We took our son when he was 9 months old, so he got even less out of it than yours will! He does like looking at the photos, though!
Planning is key. You absolutely need a stroller and you should tie a big gaudy ribbon, some fluorescent duct tape, or some other identifier on it because they all look the same when you come out of the rides, and someone will surely walk off with yours instead of their own by mistake. (We took ours with us rather than rent one there, because we wanted it in the airport, and we just gate-checked it.) You should take a small cooler that fits in the stroller or hangs from the handles so you have food with you all the time. We took yogurt and an ice pack, fruit cubes, other snacks, water, etc. We had a small fridge in the hotel, and I bought a paring knife and some dish soap when I got there, and asked the hotel for extra towels to use as a 'dish drainer'. The cooler saves a lot on food costs as well as wait times at the Disney restaurants too. With waiting in lines and waiting for food, even older kids (and adults) get cranky.
Take some new, never-seen-before SMALL toys with you than you can clip to the stroller and use on the plane or in the car - and don't get them all out at once.
Most of the people with little kids go early in the day and are there when the parks open. Then they head back to the hotel after an early lunch, take a nap, etc. while one parent takes the older child to the pool (or stays in the park for the older rides). Everyone needs to get off their feet for a while at Disney - there's an unspeakable amount of walking, standing and waiting!
We had 2 older children with us, and we knew they wanted to go on the bigger rides that not only would be scary for the little one, they had height requirements. We were staying in a Disney hotel, so we hired a sitter through Disney itself - they are bonded, insured, have criminal background checks, and so on. (I'm not sure if Disney provides that service if you are staying elsewhere, but you could ask.) She was absolutely fabulous, and when we went back a few years later, we hired the same woman again. She took our little guy on walks and stroller rides around the grounds or to the kiddie pool, she knew all the hidden gems that were free, she played with him and brought him some toys from her stash, and then we could go out with the older kids and have a real meal and do fun things so it was a little bit of a vacation for us too. I can't say enough about it.
Otherwise, be aware that little kids are often afraid of the big characters! I would skip the character breakfasts, but I would include the Epcot pavilion that deals with the sea because little kids love the aquarium fish. Just choose the exhibits carefully, watch the ones that are noisy with gunfire & explosions, and stick to the classic ones with animals your little one will recognize and not be afraid of. Take the 9 year old on the other rides during the baby's naps.
Waits are less if you enter the park and go straight to the back, then work your way forward. Everyone entering goes to the first few, so the lines start immediately. If you work your way from back to front, when you're exhausted and ready to crash, you'll already be at the exit.