You may, depending on the airline policy and your child's age.
Most airlines define an infant as less than two years of age. If your child is under two, he or she can fly as a lap infant, meaning she or he will be seated in your lap for the entirety of the flight. When adding your baby during booking with Hopper, please select whether you would like your child under two to occupy a seat or if s/he will be flying as a lap infant.
Once a child turns two, they are required to occupy a separate seat. For all children two years of age and older, you will need to purchase a separate ticket for domestic flights. You may also purchase a seat for a child less than two years of age if you'd like. Don't forget, if your child turns two during your vacation, you will need to purchase them a seat for the return leg of your trip.
The Hopper app currently does not allow booking for unaccompanied minors. Hopper defines a minor as any passenger under the age of 18 on the first day of travel. Passengers under the age of 18 may be accompanied by any passenger 18 years of age or over.