Many children are put off fish by the presence of bones (or, at least, the possibility that they might find some). They simply remember that they 'don't like fish' even though that dislike hasn't actually got anything to do with the flavour or texture of fish.
For that reason, I'd suggest avoiding anything with plenty of bones. Once you get back into eating fish, you'll probably love grilled (fresh) sardines, simply served with brown bread and lemon juice but (because of all the bones) that might be 'a step too far' to start with.
I'd also advise against starting with tuna. It's a very distinctive flavour, which some people love but many people hate. (Suggesting that you should try tuna is a bit like suggesting that someone who has never experienced any type of beer should start with Guinness! It's definitely an 'acquired taste' and probably not a good starting point).
A salmon steak might be a better idea. (There are no significant bones, it's easy to cook, and not particularly expensive). If you're not too constrained by finances though, I'd suggest either a delicate white fish, such as Dover Sole or, even better, a firm-textured fish, such as swordfish. (I simply refuse to believe that you wouldn't enjoy a swordfish steak. Look for one which is generally white, rather than grey, and grill it lightly on both sides. Serve simply, with new potatoes and lemon juice).
Chris