We went to het heijderbos a few years ago - our dc were younger than yours when we went but they loved it.
HH has a second dome that is like being in the Aztec/Jungle zone on the crystal maze, complete with real tropical plants, birds, caves, a stream, zip wires, rope bridges, climbing frames (that makes them sound like small things you have in your garden but they are ginormous) and more. It was probably a bit big for my youngest but ages 8-12 would be a great age for it if they like that sort of thing as you could have a climb with them (it’s big enough and fun for adults to go on too!), then you could sit and ‘chill’ in the cafe that’s in there while they spend more hours climbing and wearing themselves out.
The pool was nice and I seem to remember there was a snorkelling bit with tropical fish to look at too.
The food was good and not full of chain restaurants like they are in the Uk. Lots of playgrounds, nice to wander around and lots to do - and we didn’t really pay for any activities.
We did a midweek stay - the last week of the summer holidays in England is good as it is often their first week back so the prices drop a chunk.
We drove down via Calais and stopped off en route to make it a bit longer and add a different element to the holiday. We stopped at the Novotel in Ypres (Ieper)!- if you join their free club you get access to sales and we got a great price - and didn’t bother with breakfast as there was an amazing bakery over the road so went and got fresh breakfast then picnicked in our room - great fun and delicious and much cheaper than hotel breakfasts). Ypres is a great place to explore by foot, the Novotel was just off the main square and there’s the old Flemish history as well as all the WW2 stuff, which will pop up for your dc’s in history so quite relevant and interesting to see in person.
We did the same hotel stop both ways because the dc were small, especially dc2 and we thought that it would be better to go back somewhere they had already been so it didn’t seem like too many different stops but there are loads of other places to try if you wanted to find a different town or city that means you don’t have a mega drive - we found it cheaper to do this than get an overnight/long day ferry with a shorter drive in Holland.