I don't keep salamanders, but I thought I'd find you some links as someone constantly researching other herp pets - surprisingly little out there for these guys, but I think this page should have all you need to learn + continue caring for it. Salamanders are not unpopular pets, and can be pretty cool and low maintenance once you've got going.

If you have a question there's no answer to, look at other giant salamander species. There are some pretty close to that which are more popular as pets/better documented. As for the habitats, you're doing great! Check the average temperate of the water body where your kid picked them up, and as stated in that link they prefer running water. You also should try to imitate the water pH of their general area, this should be available online. In general after that, follow the guideline of any aquarium water and pick up some test strips or test kit. I assume you already have a filter for the water keeping it running, but if not your choices are 1) find a suitable submersible filter for the tank, this'll clean the water while keeping a current going or 2) build up a self-sustaining vivarium with plants and natural terrain, using a powerhead (tiny submersible motor to create aquarium currents) to keep it going. This obviously takes more work and effort, but it's super rewarding and could be an awesome project for your child if they seem into that aspect. Learning and tweaking with trial and error with my own reptile tanks and aquariums is one of the most rewarding things I've ever done! I only need to change the water twice a year - I just top it off and the plants and animals keep it pristine even with a filter turned off. It's a great idea to add smaller organisms like springtails (or similar - again imitate the habitat), snails, etc. to build up that equilibrium.

Depending on your state, you should look up if there's a list of legal/illegal pets, there's a chance they protect these salamanders since I've run into that in my province. Good luck!