I think the question is well-defined, but I can see someone misreading the fourth constraint as "twice as many children have only watched Frozen than only Tangled".
Then you would get 12 only Tangled, 16 both, 24 only Frozen, leaving 28 neither.
My problem with these "maths in real life" questions is that they don't seem to plausibly follow from any actual situation. In what movie-watching survey would you say, "OK kids, put your hand up if you've seen both Frozen and Tangled! Now put your hand up if you've only seen Tangled! Great, now, we know on average Frozen was twice as popular as Tangled, so let's work out how many have seen Frozen!"? I think kids can sense when a question doesn't fundamentally really make any sense and it puts them off a bit.
I like questions like this, they are these logic puzzles where you have some known facts and have to determine the relationships between them, and then suddenly you have an equation and you're like logic is maths and the world just feels like a better more understandable place knowing that things can be objectively quantified and "extra" information derived from certain constraints.
I also don't think the question is ambiguous at all.
