I understand tank limitations. Many of us are lucky enough to have multiple tanks or at least one large one but I totally get not being able to get a larger tank. I also understand what having a 4 year old actually invested and interested in things (I have 3 nieces, one is 4). Unfortunately, I think your tank is too overstocked. Mollies produce too much waste IMO
to be in a tank smaller than 20 gallons and Gouramiís get too large and aggressive for a tank smaller than 20 gallons for my liking at least. I know people disagree and I also know that all of a sudden taking away your sons fish would suck for both of you, but your maintenance would kill you with the current stocking/setup. My nieces love my Guppies, theyíre super active, the males are colorful, and they live well in a 10 gallon. After having Guppies, thatís always my go-to suggestion especially for smaller tanks because theyíre so much fun! I hope this didnít come across as rude, best of luck!