If regular maintenance is an issue, I'd go with a lightly stocked tank. Almost anyone can grow java ferns. They attach to wood or rocks, grow slowly & come in a few forms.
Please don't get "a" clown loach, they are shoaling fish that need friends & can grow quite large, 10+ inches!. A 55g would only be a temporary home until they get 5 inches or so in a year or 2. There are several smaller loaches that you could keep in a group of either 6 kubotai (aka angelicus), or 8-10 sidthimunki (dwarf chain) would be my first choice. Zebra loaches are a bit shy IME, cute but harder to find sometimes. Yoyos get a little big & can be semi-aggressive. Kuhlis are interesting to kids, like pretty "worms", but can be more shy than most botias, you could have 6-12. You could stay with Asian fish: pearl gouramis are pretty (1 male & 2-3 female) &. have "feelers" & a big school or rasboras either red tailed or harlequins, you could keep the danios too, if you like them.
I hear you on the breeding thing with cichlids (& live bearers). You can keep a ram or 1 kribensis (I think females are prettier) but then you don't see the fun behaviors either. I have a single angel since his friends died off, he seems ok & much less aggressive now.
I also wouldn't keep angels with gouramis. A group of corys, maybe pearl gouramis or honey (dwarf aren't often healthy, females not usually sold), the usual school of rasboras, cardinal tetras or instead maybe even roseline sharks. They are a tiny bit nervous IME, I had 1 jump from a 75g (but rescued him). A bristle nosed pleco is kind of fun for kids too, sucker mouth &, if male, bristles. You can play find the plec.
As you can see I love bottom feeders but don't get carried away. A BN & corys OR 1 species of loaches. Shrimp are interesting to kids too. The fish "should" keep the population from booming, they often eat baby shrimp if they can. Cichlids & botias (but not kuhlis) can & will hunt adults. Remember, less is easier to maintain, so pick a couple options, not all (remember less fish, less maintenance), lol. Bigger groups of 1 species can be much more interesting than several small. You can play count the schoolers...I still have trouble with that sometimes, lol.
Killies are pretty but some have special needs & for their size, can be aggressive. Do the research & ask us. You probably can aim the filter return or spraybar so the isn't too much current for them (or honey gouramis). There are other interesting options, I have a 55g planted "river tank" with hillstream loaches, gobies & whiteclouds I enjoy but it may not be what you're looking for. Another is a 20 long, all tiny fish & shrimp, can you & your child get up close & personal with your tank? I think those may not be for you but you've got fish experience, it may be time to try something very different!