Belt Ticket

so couple of month ago, I got a ticket for

"Driver failing to ensure that a passenger under 16 years is wearing a seat belt"

Although before i turned the car on, I made sure everyone had their seat-belts on, but my sister took it off to pick up a candy she dropped and at that second, an OPP officer was standing at the Cambridge 401 exit doing seatbelt check and give me a 110 ticket. I took it to court but they didn't really care and convicted me anyways. I was really pissed off? anyone had this happened to them?