My kids totalled ~50# maybe when I started pulling them in a trailer and I pretty much hit a fitness wall taking them home w/ my bike geared 52/42/30 and 12/25 9 spd. I wound up switching to a mountain bike for the low gears to pull them home up the hill. Now they total ~60#, and while I have down-geared my main ride (28 tooth is now largest rear cog, and am planning to drop the 30 chainring for a 26), I definitely need the mtn bike to pull them up the slope our house apparently sits atop, esp. if carrying groceries or panniers full of work stuff in the trailer as well as the kids. I mean, I can physically pull them home in whatever gear I have, but if I need the energy to keep up w/ them once I get home, I need low gears on the bike.