I second the cory idea. Bristlenose plecs stay on the smaller side (~4-5in) but produce alot of waste and IMO
need a tank bigger than 20g. They also hide alot, so wouldn't be out much (same with most plecos).
Cories, in a big enough group (6+) would be out alot and they are very cute so your daughter would probably love them.
You could add a couple more neons, since she likes them, and then get a group of 6-8 cories of the same type. There are tons of cories to choose from. I'd go with a type that stays on the smaller side, if you can. Panda cories are awesome and they stay around 2 inches. Also, false Julii cories or Leopard cories (corydoras trilineatus) if labelled correctly stay around 2 inches. Others are Bandit cories (Corydoras metae) and Skunk cories (Corydoras arcuatus).
Any cories will do, but some get over 3 inches...so with the smaller ones, you could have a larger group IMO