Once tax season is over, you can go back to being the only child.
Your taxes can be reduced in the US if you have children: by claiming more dependents on your tax return, you get to pay less to the government, which everyone tries to do. Here, the man is humorously bending the tax rules by treating the family dog as a human child to save tax money. The human child is not happy about it, but the parent assures the kid that it's only for tax season that they need to do this charade.