I'd add a few more at a time now.
Would be fun to go to the pet store for the kids
and pick out some really pretty ones.

You might consider setting up a quarantine tank.
I've never done it, but if your kids are really fond of
the first residents it would be a way to safeguard
them against any illness the newbies might bring
in.

Keep your stocking level low and feeding level spare
and you'll hopefully avoid any really bad green water
problems, especially if you've gotten carried away with
plants which it sounds like you have! :-)

kathy :-) www.blogfromthebog.com
this week ~ a snake story

Pond 101 page for new pond keepers ~
http://hometown.aol.com/ka30p/myhomepage/garden.html