Front pannier and child-seat balance
I have ridden with children about that weight on the back; there is no way I would consider adding more weight to the rear of the bike. If you worry about the effect of front panniers on steering, I can only presume you have experience riding uni-cycles if you are considering adding rear panniers.

An option to consider would be panniers on the rear and a child seat on the cross bar. This provides many advantages over a child seat on the back, however it requires a largish bike frame - something I did not have.