Pannier rack bag for a not suitable suspension fork (Btwin Rockrider 5.1)
While I am not familiar with your bike or its front fork. I would think a front pannier rack like this may work. http://www.zefal.com/en/racks/118-raider-front.html 

Please note that weight on the front fork makes the bike even easier to fall over. Please be very careful when the baby is on the back. I am not a fan of rear baby seats and would just buy a trailer.