Hey Ross,
I would get some turbulence in there at the surface of the water either an air stone or power head to promote O2.
I assume your tank is cycled so...
It will not hurt to do another big water change...like 70% and another in a day or two just be sure you use a good dechlor product. Vacuum each time and stir up the bottom with your hand so you can siphon out as much gunk as you can. Keep changing out your filter media, new activated charcoal every few days will help with the smell.
Closer supervision of the child, and/or your friend, whichever is more appropriate

LOL it takes me back but I bet it has happened to a lot of us.
Regards,
Jay