I've just got back (this week) from a trip to DLP with my 6 year old DD. We've been a few times since she was 2.
I feel by now I'm pretty experienced (been going myself since I was 8) but as DD has aged the trips have evolved each time. We always have a blast together.
I've stayed on site and offsite and travelled pretty much every possible way so here are my top tips if you go:
Go term time while you can but to be honest, we've now been going in school holidays for a while and you can still have a good time.
Travelling by car is great because you can control the time you arrive, you can take more stuff but you might want to plan an overnight stay on the way down - we live in the Midlands and usually stay overnight in Dover before a ferry if we drive. I prefer to travel by eurostar but it can be costly if you book via disney to their train station as there is a change in lille, it can be stressful to deal with getting luggage on and off and having it with you all the time. Flying can work well but queues at the airport are chaos at the minute due to the new passport control changes, we queued over an hour for passport control on arrival and leaving this week.
Staying onsite allows you to make use of the magic hour in a morning, which is great for some of the busier rides if you go straight there at park opening. Also some of the hotels have a character in them so its a great way to meet characters if your littles are into that (we only queued around 15 mins a morning). I usually stay in the sequoia lodge because I love the theming.
Bookend the day, particularly with little ones. Go to the parks first thing, dip out around 1-2pm and head back to the room for a nap/rest and a swim if your pool has one. Then go back to the parks a little later for dinner and rides. Some of the popular rides queues tend to drop off a little in the evening making the queue more palatable. Staying onsite makes this easier Personally but I have done this staying offsite too.
If you want to watch the nighttime shows but know your kids wont make it to 10/11pm in the summer go in the winter because it will be dark earlier and the parks tend to close earlier.
If your kids want to meet characters and you can afford to, character dining (particularly royal banquet) is a great way to do it without queues but it does book up early so try and add it as soon as you can.
Decide your budget for souvenirs before you go. It can quickly get expensive if your kids want disney ears/bubble wands/shoulder buddies. Personally I try to take ears and bubbles from home, though each time we've gone i have let her pick one different one of these.
Snacks, there are lots of snacks there but you can also bring your own from home or buy some at the supermarket in val d'europe. There are water fountains dotted around so take a flask and some squeezy squash and you'll save a fortune on drinks, our hotel had an ice machine so we filled our bottle with that each morning to keep it cool all day.
There are a lot of parades and shows. It isn't likely to be possible to see them all but choose the ones you think will be a hit and then don't tell your kids about rhe ones you're going to miss.
For me, I think 3nights/4days is the sweet spot particularly if you are arriving on day 1 morning and leaving on day 4 evening but I also think if you go for 5 days and go into the park for 3 full days that can also work but this tends to be easier to do offsite as the park hotels include tickets for all the days. We never use Premier access and get to do everything we want to do in this time, the only rides we didnt do this time are the ones she's not tall enough for, Peter pan and orbitron (she's not bothered and always a huge queue), we did her favourite rides 3-4 times in that period watched both night shows and in terms of parades only missed the main parade because we've seen it before.
I hope that helps!