Clown Gobies are very prone to being sensitive and shy eaters. It all depends on your LFS
, how well they've been handled and what they've been through before you get them.
That being said, if they are in a tank by themselves and you are diligent the first week or two on making sure they are eating well, they quickly become hearty eaters and very unique little fish.
I think the best thing for smaller tanks like this is to just go zoo crazy. You can get an insane amount of color and diversity for very little money, and it will really show the diversity of ocean life to your kids. You might even find you really enjoy having a small zoo "zen garden"
If you were looking for additional fish/inverts to add, sexy shrimp are great nano inhabitants, and are always very fun to watch.
Good luck and keep us posted!