Well the weight of the doll multiplied by gravity is the force acting downwards. So say it 0.5kg heavy, its force downwards would be the mass, 0.5, timsed by the force of gravity (9.8 in Mechanics or 9.81 in Physics) which gives a downward force of 4.9N, I think.
I'm not sure how you'd find the upward force considering it's helium, look on the internet for some research.
Hopefully, this has been some sort of help.