.You are doing that.
It's unavoidable because the term is only found here. So you will do it as well if you exegete the passage.
Repeating something false does not make it true. “Paul” never directly applies νήπιός to Jesus here. Even an indirect application to him would be hard to justify.
What I said was:
"The fact remains that in the culture of the time where and when Paul wrote Philippians and Galatians a child that was not of age was considered δούλος."
I don't maintain that it is applied to Jesus at Galatians 4.