try this microsoft page for some tips on how to securely set up your computer.
here:- click here
for "Using Windows SteadyState" Steady state can be set up to restrict and log use, as well as enable you to return your computer back to normal after children change settings, easily add each child as a new user account and set the controls for each account, then when thay do get on your pc hopefully they cannot do to much damage and what is done is reversible and does not effect your own files, settings etc.
for some freeware tools try this :-click here
"SAFE LOGON" and "Windows security officer",
will both upgrade and enforce rules regarding usage what is done, how long, time limits, etc
"windows security tweeker" also looks very good.
there are some commercial products as well,