M.P.
Well, it sounds like your youngest is ready to start. My son was alittle over 2 when I started with him and he was done in 1 week. I think alot of it has to do with timing and making sure that the child IS ready to start. First, I recommend NOT using anything like pullups (except for naps/bedtime). When we ran out of his diapers I decided it was time and that was it. I went out that day and we bought him some big boy underwear. I put his little potty in the hall way, outside the bathroom door. I let him play as usual but I kept a close eye on him and kept asking him if he had to go. If it had been a while I would congratulat him for still being clean and that take him to the potty to "try". I always made a huge deal when he went on the potty but I also made a big deal just for him being dry/clean (even if it was only a few minutes) I think alot of praise and encouragement is the key. I would walk into his bedroom ever so often and just say, "are you still clean". He would smile and say yes. Then we would high five, clap, dance what ever! The first 2 days he had 2 or 3 accidents a day. I did use pullups but ONLY at naptime and bedtime. We called them his nighttime underwear. By day 3/4 he was starting to understand the signals his body was giving him right before he was wet. During these days he would start to pee in his underwear, stop himself and RUN to the potty. We still had to keep changing his underwear but he was doing great. By the end of the week he was completley trained with NO accidents at all. During this time I also never used pullups when we left the house to go somewhere. I just made sure he peed before we left, took a plastic bag, towel and change of clothes. I feel that it diminishes their confidence if you don't try to trust them out of the house as well. It tells them, "I don't think you can do it" We NEVER had an accident out of the house. I think it was only day 3 of potty training when we were shopping in Target (farthest part of the store from the bathroom) and he started yelling and dancing, "I have to PEE" We RAN as fast as we could and he did make it. We had a party right there in the store and I ended up getting him a new toy because I was so proud.
The bottom line is....once you start, be consistent, have patience, give lots of praise and encouragement, use regular underwear NOT pullups (they are just fancy diapers) and make if fun!
Good Luck and Happy training :-)