MC, virtually ALL tours are "skip the line", as are timed-entry tickets for the Colosseum. This means you skip the TICKET lines but not the security-check line-ups. No one skips those, although the line for group tours can move along nicely.
As far as which guided tours access which locations, you'd have reference the information for each tour. Some tours do just general areas of the Colosseum, some include restricted areas of the Colosseum (such as the underground), some include the guide only for the Colosseum but not the Palatine or Forum, and some include the whole enchilada.
As you've been to the Colosseum before (did you also do the Forum +Palatine?), you should have some idea of how much of it you want to cover? I do recommend a guide for the Forum and Palatine as they are complex sites - more complex than the arena - which benefit from narration. I personally feel they're well worth seeing but not everyone is all that jazzed about ruins, especially if there's not much left of the original structures.
I'd also keep the stamina/attention span of your youngest in mind? Tours which include the whole enchilada can be quite long - at least 3 hours - so whether he/she can hold up and stay engaged up is a factor. Cost also plays into the equation: tours booked through Coopculture are going to be more economical than those booked through private services, although they can also be more difficult to snag as they tend to sell out very, very quickly when they become available.
Edited a typo.