Do you have ANTS in your pants?

Ants in one's pants:  to be

so excited, nervous or anxious about something that it's hard to be still and calm.

Ants are very small insects that often sting. If ants were crawling around inside or on your pants it would be very difficult to sit still and be calm. Instead, you would likely jump around and try to get them out of your clothing as soon as possible.
Notice that this idiom describes a kind of excitement that can be either positive (excited about something) or more negative (anxious or worried about something).

Examples:
— My children have ants in their pants because tomorrow is Christmas and they are excited about their presents.
— Every time we take our children to church they jump around like they've got ants in their pants and I constantly have to ask them to sit down.