I have two children, they were 3 & 5 when we first went in 2011. We had a great holiday and they both (obviously) enjoyed it. When we would reminisce my youngest new what we were talking about through pictures but not so much through memory.
We went back last year (2013) when they were 5 & 7, and had an even better holiday! It was great not to have a pushchair and my youngest wanted to much more then he wanted to do before. We had initially held back telling they were going then did a big reveal for their Christmas present, I'm not sure if they were more excited or we were! This time we stayed on site rather then offsite and had some extremely magical moments (being in the magic kingdom parade, getting lots of pixie dust thrown our way
)
Two months after we came back we all had massive disney fever and dh and I made the decision to go back in 2014, this time we told the boys from the start. They have been involved with different decisions we have made and have helped mot decided which adrs we will book. They are excited to be going and can't wait to do all the things they loved before, along with some new bits. We are again going to stay on site, but initially I wondered whether because we had such a magical time last year if it would be overkill by going this year as well. My fears have been demolished though as I have seen how much my two want to go back, there reasons are not for the extras we did,but instead for the characters, rides, swimming pools etc.
What I'm trying to say is that, each holiday will be different. As they grow older they will want to experience different things, along with their old favourites. Dh and I have said that we will continue to go as many times as we can afford and they boys want to go. We emphasise how they are extremely lucky to go as it cost salon of money and we also say that we can't do other things in the uk as we are saving for America and I think they understand that it is a treat to go there rather then becoming I complacent about it.