From experience you have to monitor what they are doing and who they are chatting with. Having them have a restricted account is a good start. This may sound draconiun, but don't allow them to be online from their bedrooms, I apreciate that maybe difficult. Supervision is the key to keeping them safe though. So the ideal would be for them to use a family room. Left to their own devices they will learn from there on line friends to bypass what ever secuity you set up.

Make sure you lock them out of the bios setup by setting a password. Install some thing like netnanny or otherparental control softwre to protect them from inapropriate matterial.

I can't help with the best IM client, I simply don't know which is the securist......personaly, i think they are pretty much all the same, security wise for young children.

Educating them to be wise to the dangers of the net can be a bit of a battle but is very important. My eldest daughter is quite adept at spyware removal now but it's taken me about two years, to get through to her that all the joke emails and sites with games on that her friends point her to are not allways friendly.