I transport my 15 Kg kid on a child-seat. I want to set a pair of panniers. Even if I could put them on a rear rack (I don't know if I can), would it be a good idea to choose a front pannier system aiming to balance the weight? I mean to do not have almost everything in the back part of the bike.
Without panniers my front wheel often takes off from the ground on slopy rocky paths. Is it worth it, knowing that I will lose some steering comfort with front panniers?
I've ridden "fully loaded", with substantial load in the front panniers. It does affect your steering, but you get used to it after a relatively short time. (And when you take the bags off the front the bike feels like a sports car!) It does require a little more muscle to steer, but in a way steering is actually more stable.
You do need to be careful of toe and knee clearance when loading the front. Not generally a difficult problem, but it's bad to discover you have a clearance issue while rounding a corner at 15mph in traffic.
If your front wheel is lifting off the ground, then I'd suggest keeping the weight forward as much as possible.
Front panniers are the way to go if you can do it. As you say, it spreads the load. In fact I often feel better with panniers front and rear than with either on their own. We use "low-rider" front panniers on the tandem and often have our 10kg child on the back and this seems pretty comfortable and controllable, but obviously a tandem is quite different in geometry.
You could also consider a bar bag if fitting panniers is tricky, but this generally provides less space and the weight & windage is higher up, so possibly less stable.
You'll need a bit more air in your tires to avoid pinch flats with the extra weight (especially the rear).
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.
