A kid was asking a mother for money.
Mother: Sorry, I don't have money.
The kid kept asking the mother for money.
Mother: I already told you I don't have money.
The kid (the middle child): I'm your fucking child!
Listen up, buttercup! This joke is about a middle kid feeling overlooked. The mom keeps saying she has no money, and the middle kid's like, "Hello, I'm your child!" Basically, they're hinting that they're often forgotten or ignored compared to the other kids. You probably understand that feeling!