Hi, given the lens you already have i would get the 24-70 2.8L or the 17-55 2.8IS. Neither of these lens are small but honestly if you want to take pics indoors without a flash then you really do need F2.8 at a min. I started out buying F4 zooms and very quickly discovered there limitation indoors. I use the 24-70 2.8 on a 50D and its perfect for indoors/kids etc. Having 50-70 and 2.8 is very useful for getting closeups of kids indoors where as i am pretty sure you will find the 100 2.8 is too long for most indoors stuff. If you opt for a zoom that is slower than F2.8 then your other option is to add a fast prime but that means yet another lens and with a fast prime your depth of field can be very narrow. Rather than use a fast prime i prefer to use a flash if F2.8 isn't fast enough because i prefer more depth of field. The 15-85 is nice lens and is much smaller than the 17-55 or 24-70 but indoors your going to find yourself at ISO 1600 + very quickly.