I've just had a look to see which hotels are on the Splashworld site and we haven't been to any of those, but we have been to a couple on the Thomas Cook Waterpark site - the Royal Dragon in Turkey and the Caretta Beach Hotel in Greece. I would highly recommend the former - a lovely hotel with some good slides and excellent accommodation and food. The Caretta Beach also has a very good water park but the accommodation is VERY mixed - some nice rooms but some absolutely tiny and basic (guess which we got....). Food is also nowhere near as good or as varied.
The best waterpark hotel we've been to was the Lykia World one at Ou Deniz in Turkey but I think it's changed hands/name(?) since we went a few years ago so can't vouch for it now.
We went to the Splashworld in Lanzarote and absolutely loved it. My 7 year old loves waterslides but some of the bigger ones were too scary for her, it didn't matter as there were plenty of other areas for her to play in with our 4 year old.
What we found was the evening entertainment was not as good as say a Holiday Village and while the repetitive acts were loved by my 2 this my not tick the boxes for yours with them being a bit older.
Nevertheless, we would go back in a shot and the 'Octopus Park' is still a topic of conversation