We love Maui!!! We have been there four times, twice with kids similar in age to yours. My husband likes it better than Oahu because it isn't as crowded. Don't get scared away be web sites that say Maui is not for kids. The weather has been fabulous when we have been there in February, March, April, and July. In all of our trips I think we have had rain two days and both times it lasted for a very short time. July was a little hot for my liking, but the spring has been perfect weather!
We always stay in Kihei. I think it is a little cheaper than staying in Ka'anapali and definitely cheaper than Wailea. We stay at the Maui Banyan which is a condo/timeshare. For us that is a better deal. We like having a washer and dryer (don't need to pack as much) and a kitchen (saves lots of money on eating out). We probably ate out for breakfast once and dinner 3-4 times not counting the luau. It was also nice to have the kitchen so we could have lunch things to throw together on beach days.
Maui Banyan in just across the street from a great beach (about two blocks if you are at the far end of the property). They also have two pools that also have barbeque areas. I think the Hyatt in Ka'anapali might have a kids' program. I think they have penguins and flamingos on site. The Four Seasons in Wailea has a kids program, but I'm guessing it is pretty expensive. Those hotels tend to have just one restaurant on site and they are very spendy! We don't use kids' clubs or babysitting so I don't know what their programs are like. I'm just not comfortable with leaving my kids with total strangers!
The beaches in Maui are great! All of their beaches are public and well maintained. We like the beaches in Kihei. They are good for swimming, snorkeling, and wave jumping. There have been times in the morning and afternoon that we are about the only people there. Big Beach and Makena Beach farther south are great too. The surf at Big Beach can get pretty big so it isn't the best for kids to swim. There is a Baby Beach in Lahaina that is pretty sheltered and a great beach for kids. There are several shops on the island where you can rent beach gear (umbrellas, chairs, snorkeling gear, etc.) for the week for pretty cheap. We have also bought cheap umbrellas and then passed them on to people just getting to the island.
The Maui Ocean Center is a nice little aquarium that can fill about 1/2 a day. In that same area there is a mini-golf place that also has trampolines and I think bumper boats. We have never gone there.
If you are there during whale season you must do a whale watch with Pacific Whale Foundation. We have never had a disappointing tour with them. They also have dolphin tours and snorkeling tours if it isn't whale season. The crew is great with kids! At the end of the tour they always do a kids' program that is really good. The bay that the tours leave from is the windiest in the world, so the ride can be a little rough. If kids tend to get sea sick use the motion sickness bracelets. We didn't use them on one tour and everyone got a little woozy. Used them on another trip and had no problems.
Luaus are great fun! Our favorite is the Old Lahaina Luau, but it is the most expensive and not the best one for kids. Last time we went to a luau at the Hyatt at Ka'anapali Beach. They have a kids' buffet with kid friendly food and I think the kids were free with paying adults. The kids loved the fire dances!!
There is a lot to see in Lahaina. Our kids loved running around the HUGE banyan tree at Banyan Park. Across the street there is an upstairs shop (Dan's Greenhouse) where kids can feed exotic birds and pot bellied pigs for a small fee. There are a lot of shops in downtown Lahaina, but a lot of them are art studios and jewelry stores that are not all that kid friendly. Walking on the sidewalks can be a pain because it gets really crowded (especially if there is a cruise ship in port), but there are a lot of fun things to see on the street. My kids loved watching the crabs on the rocks. There are also some good family friendly restaurants, ice cream shops, and shaved ice stands. There is a huge Hilo Hattie store that is a good place for souvenirs. There is also a mall where they have free hula shows. One day a week they have one specifically for kids.
There are some swap meets on the island, usually on Saturday, that are another good place for souvenirs. There is a daily swap meet type of setup in Kihei and a couple of outdoor shopping areas that are lots of fun for kids. They also have a farmers' market where you can buy yummy fresh fruits. Just be aware that unless is it pre-certified you cannot take it back on the plane. We have had to throw out fruit (or stuff ourselves full with it!) at the airport because they wouldn't let us take it on the plane.
Our next trip we are planning a day at Iao Valley State Park. We haven't been there yet, but we have heard great things about it! There are easy hiking trails, you can swim in waterfalls, and there is a nature center and a gardens. We also want to visit Surfing Goat Dairy sometime. There is a Sugar Cane Train that you can ride, but we have heard mixed reviews about it and it seemed a little spendy so we have never done that. We would also like to spend a day in Paia, but I don't know how kid friendly it is. It sounds like a pretty little bohemian town.
With a 4 year old I would probably skip the Road to Hana and the drive up Haleakala. They are both beautiful drives, but a lot of time in the car!
Our last trip looked something like this: Day 1--Beach and exploring Kihei, Day 2--Aquarium and Beach, Day 3-- Whale Watch and Pool, Day 4--Lahaina, Ka'anapali and Luau, Day 5--Beach all day, Day 6--Lahaina and Beach and Pool, Day 7--Beach, souvenir shopping in Kihei, and off to the airport (you can only fly out of Maui in the evening)
Definitely get the book Maui Revealed. It is hands-down the best travel book we found for Maui. It was written by people who actually have lived in Maui so there are lots of tips about lesser known places and places that are better to avoid.
Maui is a little bit spendy, but we have never regretted a trip! Our 4 year old asks often when we get to go back to Hawaii! They want to learn how to surf when we go next year!
As a side note, I personally would avoid Mexico. I have a co-worker who has family in Mexico and they have told her not to come and visit. They have even told her to avoid the tourist areas too. They have said that it is just too dangerous right now to travel there. A high school classmate of mine was murdered there a couple of years ago on his anniversary trip. They were just outside one of the biggest tourist areas. If you choose Mexico, go with an all inclusive, private resort and don't plan on leaving the resort property. And be extra vigilant even there.
I don't think you can go wrong with Maui!