We loved Efteling, mine were 7 and 5 when we went. We went to Disney Florida a few months afterwards and we had a far, far better time at Efteling. Less queues by far, better food and lovely staff.
(We drove from the ferry at Calais so can't help you with Amsterdam airport.)
I watched a lot of YouTube videos about it before we went so I had a good idea of what was there and what the DC's would like so that I could plan the trip. I'd really recommend doing that. I also looked up the height restrictions on rides.
A lot of what they remember isn't the big 'rides' but all the little unique quirky stuff, like the donkey that poo's gold coins, or the wolf hiding in the bed in the red riding hood house, or the edible straws that came in their drinks at the pancake houses,
Food is absolutely amazing for a theme park. We had lunch in Fabula and we had salmon fillets with new pots and veg.....we were fairly flabbergasted you could get 'proper' food and not nuggets and chips.
We had dinner in Polles Keuken and let the DC's have sweet pancakes as a main, they were massive so there was no way if they'd eaten a savory one first they would have managed. But the theming of it with the moving ceiling stuff was wonderful.
The water/music show at night was surprisingly good, and a nice way to end the day.
Favourite rides for us were Symbolica, which was charming and gentle and very efteling! And Froomvlucht which was completely eccentric but lovely. Plus the fairytale forest which is so out of the ordinary for a theme park, the DC's would run ahead to see whatever house was next and who was in it, and also the carousel park which sounds odd, (lots of different old musical carousels together, but in person its lovely).
(Everyone hated the Cursed house. The whole 'story' part of it is in Dutch so we didn't have a clue what was going on, and when we got onto the ride part the DC's were quite freaked out when the walls started moving, so I'd think carefully about that one or at least warn them its actually a moving ride disguised as a story time)
I think our tickets were about 20% cheaper because one of us had a birthday in the month we went, dont know if this is still a thing but when we went, if you sign up to the newsletter they ask you the birthday dates of your family and then send you a discount for each person's month.
We stayed in the Hotel de Kroon in Kaatshuvel, which was about a 7 min walk with little legs(!) and everyone in the hotel ate breakfast and then walked on mass into the park.....was a nice place that had a family playroom bit so all the kids played together. (When we went you had to book a minimum of 3 nights to stay in the park itself and we couldn't stay that long so we stayed outside.)
Oh, last thing, they have these little wooden carts near the entrance that you pull along, think it was about a 5euro deposit, ours were too big to need one but they are so useful to lug everyones bags around in, worth getting one!