if you insist on putting their seat on the bike, I'd suggest one of those seats that sits just behind the handlebars rather than the kind that goes in back.
I note those copilot Taxis are /max/ 40 lbs, if your kid is 30 lbs now, he'll outgrow that in a year.
awwww....
(from
http://www.amazon.com/WeeRide-Kangar.../dp/B000FIH0EG )