Babies don't magically change weight. It's just when they are fighting being picked you are probably being forced into a position with less mechanical advantage.
It doesn't seem to make sense logically, if you think of weight as mass * gravity. But hypothetically speaking, if the baby can do this -- https://www.youtube.com/watch?v=f5Wks63Ng0Y -- don't you think the weight will be different to whoever is picking the baby up?
We don't pick up the baby as a single movement. If we did, then yes there can be no magical weight change. But in reality, there's a bit of a choreography involved. Perhaps the baby holds up its arms, you put your arms below its shoulders, you raise it slowly. During this process, surely it can control how much of its weight you perceive?
You're right, just using the wrong terms. The baby's weight is fixed at mass * gravity, it doesn't change. What changes is the force required to accelerate the baby upwards, which is the delta between the force of gravity (the baby's weight) and any upward forces opposing that gravity. So if the baby pushes upward with it's feet to help you, you have to exert less force to lift the baby.
Scientifically speaking, the weight of an object cannot change unless gravity changes or it's mass changes. The forces required to accelerate that object in a particular direction vary wildly based on all kinds of things that exert a force on the object.
Off topic, but that does make me curious how "weight" works in an environment where you're exposed to more than one gravitational field. Like if you're close enough to two black holes to be affected by the gravity from both.
We don't pick up the baby as a single movement. If we did, then yes there can be no magical weight change. But in reality, there's a bit of a choreography involved. Perhaps the baby holds up its arms, you put your arms below its shoulders, you raise it slowly. During this process, surely it can control how much of its weight you perceive?