I do not plan on paying to have this kid join in on having dinner with us.
Why will you not pay for the kid?
I just do not have the money to be spending on random kids.
If it is just a child, you should pay for the child to eat. It is a nice gesture and shows that you care.
Nah, I think I won't pay.
The kid is not your responsibility so nobody is forcing you to pay for the child. It would be kind for you to do, however do not be upset if you simply can not afford it.