x2 on the rock location, is it against the glass? also i would go with at least 30lbs of rock for a reef, you can get smaller "filler" sized rock to fill in the gaps on the rocks to hide the seams in the larger rocks to make it look more natural and also add a few more pounds of rock to your tank without adding a huge chunk on top making the scape look odd.
you can also get a small skimmer for your tank like the aquaticlife 115 i ran that one on my 29g bedroom reef tank and had great success with it so on a 20g it should do even better, it pulled consistent dark skimmate and my water parameters were always spot on.
for lighting that t8 is not going to support corals your going to need a good light, i suggest the 120w dimmable led
fixtures, im running two of them on my 75g reef and i have sps
with no trouble at all, plus it gives you full customization over the color of light you want in your tank, more white, or more blue, or full white and blue, its all up to you, the light covers 24" of water and about the same depth of water, so one fixture would be perfect for you.
link--> Aquarium Protein Skimmer: AquaticLife Internal Mini Skimmer 115
link--> 120W Dimmable LED | eBay
as far as livestock, its all up to personal preference but theres plenty of pretty colorful, hardy, and active fish you can add to you tank, that you and your children can enjoy, heres a few of my favorites ive had great luck with and are very hardy fish, plus they just "pop" in a reef tank, active and playful, plus theyre colors are bright and striking.
Saltwater Aquarium Fish for Marine Aquariums: Ocellaris Clownfish, Tank Bred
Saltwater Aquarium Fish for Marine Aquariums: Bicolor Dottyback
Saltwater Aquarium Fish for Marine Aquariums: Purple Dottyback
Purple Stripe Dottyback
Saltwater Aquarium Fish for Marine Aquariums: Firefish
Saltwater Aquarium Fish for Marine Aquariums: Firefish, Purple
Saltwater Aquarium Fish for Marine Aquariums: Six Line Wrasse - Reef Safe Wrasses
Saltwater Aquarium Fish for Marine Aquariums: Tanaka's Pygmy Wrasse, Wetmorella tanakai