I’ve been to a few of them, my kids are 6 and under.
We’re just back from Lalandia and my children loved it. I didn’t think it was too expensive, guests can buy gold wristbands so kids can do almost every activity for free. I need a holiday after the holiday as it was exhausting - they wanted to swim/bowl/trampoline/mini golf etc every evening after a full day of activities, I don’t think I sat down the whole time 😂 the houses are lovely and modern but I thought the centre needed some TLC. Legoland is nice and then you have Lego House which is a fab day and Wow Park is brilliant.
Efteling is beautiful but we only found playgrounds outside the park. Our houses didn’t have access to the swimming pool. Kids had a great time but it was all about the park itself.
Europa Park is brilliant but I didn’t think Rulantica was great with younger kids - the wave pool was pretty fierce and the slides, lazy river etc all had waits. It was too warm to do much outside (it was 37 degrees both times we were there) and lots of the slides are out doors. I think it was expensive for the water park tickets. Europa park itself is brilliant and there are some cute places nearby.
Another option might be Village Nature and Disneyland Paris. Village Nature is a Centre Parcs. I’ve only been to it (no other Centre Parcs) but I think it’s a a bit more basic than the UK ones. My kids love it, fab swimming pool, lots of activities and Disneyland is 10 minutes away. Easy to visit Paris by train as well.
Hope that helps, we’ve been doing a bit of a theme park tour this year!