The momentum of the mom and son together is 10,000 kgm/s.
Momentum is a measure of an object's motion, and is calculated as the product of an object's mass and velocity. When the mom picks up her son, the velocity of the combined system doesn't change, so the momentum of the combined system remains the same as the momentum of the mom alone.
p = m * v
where p is momentum, m is mass, and v is velocity.
For the mom alone:
p = 10000 kgm/s = 10,000 kg * 1 m/s
For the mom and son together:
p = (m_mom + m_son) * v = (10,000 kg + 20 kg) * (1 m/s) = 10,020 kg * 1 m/s = 10,020 kgm/s = 10,020 kgm/s