Sounds like eventually you'll need a fairly comprehensive tool kit. Can I recommend the
Park Tool website as a good resource. Their site has a good how to guide which tells you what tools you need for each job. (Theirs aren't the only bike tools out there by the way)
General tools I'd suggest to start with would be:
A set of ball ended allen keys (up to 10mm)
Screwdrivers - a selection with flat and Phillips blades
Chain splitter - will be needed to size new chains to fit your bikes
A decent set of spanners - a range of 8mm to 17mm should do the trick. (Make sure whatever set you have includes a 15mm. If the kids bikes don't have quick release wheels this is the size the wheel nuts are likely to be).
Cone spanners - for adjustment of cup and cone wheel bearings.
Cassette tools - You'll need seperate ones for Campag and Shimano. (There is a version of the Shimano cassette tool that will also do the lockrings of their brake discs. If any of your bikes have Shimano discs it would be worth having this model).
Chain whip - also needed for when removing a cassette.
Freewheel tool - it's quite possible the kid's bikes will have old style screw on freewheels rather than cassettes. If you have a bench vice, clamp the tool firmly in it and turn the wheel or you can use an extending wheel wrench and appropriate socket.
Crank and bottom bracket tools - sounds like you'll need several different sorts. The Park Tools site will be good for confirming which ones you need.
Cable cutters. You can get away without these but they make replacing cables so much neater and easier than trying to use side cutters.
Hopefully I haven't missed anything. I'm sure others will be along to add more suggestions.
