I don't know if I would recommend long finned danios. They are very active and may get frustrated in a smaller tank without the room to zip around the way they like to do...they NEVER stop !!
Betta, definitly..they seem to be the perfect fish for smaller tanks. They do sell a jr heater at walmart for smaller tanks....costs about 6$ Make sure you have a thermometer in there to keep an eye on the temps...those smaller heaters don't have regulators on them...you'd have to watch it the first few days to see where the temp settles before you get any fish to put in it.
Oh, no tiger barbs in with bettas...if you do get a betta, 2 small corys "OR" a couple of otocinlus will do well with him.....or....some african dwarf frogs, bet your son would like that. Only trouble is feeding the little guys. The betta most likely would snap up their food. I have to hand feed mine to keep the fish from eating it all.