Like your child’s teeth, retainers need to be cleaned with toothpaste. All they need to do is brush gently, and be extra careful around their wires.
If they have a lower fixed retainer, they will also need to be careful when brushing the wire. They will also need to take extra time to brush the teeth around that area, as food can get stuck their more easily.
Here are some other pointers on keeping their retainer safe: