I did my first commute with the little one on friday. I got a used topeak babyseat off craigslist, but was actually going to get a iBert front bar thing before wife veto'd it.
I found most only hold kids up to 40lbs and after that you need to do a trailer which when i was looking all held up to 100lbs (so two 50lbs kids or one fat kid).

once your rolling it doesnt affect my balance and at stops and starts you feel the weight but nothing that will change your habits.

but i do ride slower not due to the weight just a subconscious safety thing knowing i have precious cargo.