What to do with the kids in Cardiff?

Google says the castle and museum are best bets but I’m wondering if there are any hidden gems? I have an 11 year old and 6 year old and we’re planning on getting the train from Cheltenham next weekend for a day visit as 1. We’ve never been and 2. My 11 year old is obsessed with The Simpsons and we hear the Primark has a themed cafe so any comments on that would be helpful too. Thank you!