buckle in
To anchor or fasten oneself or another in place with a seatbelt, as in a vehicle. A noun or pronoun can be used between "buckle" and "in." Can you buckle the baby in while I put our bags in the trunk? This car is not moving until everyone is buckled in!
buckle someone in
to attach someone securely with a vehicle's seat belts. (This includes airplane seat belts.) Don't forget to buckle the children in. Did you buckle in the children?