We have been twice and travelled on Eurostar both times. It's a great way to get there because the station is right outside Disney.
If you do travel on Eurostar, book the Disney express (an extra £16 per adult I think) and you can check your luggage in at the station when you arrive. They will then give you your park tickets and transfer your luggage to the hotel for you to collect when you check in.
Pack some snacks (cereal bars/apples etc) to take into the park with you each day. Book the (one each per day) meal vouchers and stay in a hotel that does a buffet breakfast. Fill up in the morning at that, then use the vouchers in the evening for another good meal. We did this both times and lasted all day till dinner.
Don't buy bottles of water inside the park. Go out to the station and buy them in the newsagent shop. Much cheaper.
Make sure you take comfortable shoes, you will do a LOT of walking.
Get hold of a couple of maps and schedules. We just went where we fancied on the first day then worked out what shows we didn't want to miss and worked out when we were going to see them. In 5 days we still didn't do everything!
If you go to the Cinemagique show be aware that it's a really good show but there are a couple of split second shots that might scare children.
Don't get stroppy if your children miss out on meeting characters when they are walking round the park. People will push in, characters will have a limit to the time they are in one place. If you get grumpy about this it will make you look like a right (yes I am looking at you Mr Stroppy, I'm pretty sure it was your miserable attitude that meant the character your son desperately wanted to meet went off before he had chance).
If your children like the princesses go to the Princess pavillion. You will queue for an hour (or possibly more), it will be a frustrating wait but when your children see their favourite princess it will be so worth it. Note - when you get to the end of the queue there will be 2 different princesses. You will only meet one but if you go back be sure to tell them at the end of queuing if you've met any already and they will try to make sure you get a different one.
Enjoy!