passcode issue

I HAVE AN OLD IPHONE 7 THAT I WAS LETTING MY NOW 5 YEAR OLD TO USE FOR CAR RIDES TO PLAY GAMSE ETC SINCE I GOT A NEW PHONE. WELL I HAD A PASSCODE SET THAT HE KNEW SINCE WE USE THE CODE FOR EVERYTHING, WELL SOMEHOW HE FIGURED OUT HOW TO CHANGE THE PASSCODE AND OF COURSE 5 YEAR OLD HE DOESNT KNOW WHAT #'S HE USED TO SET IT, IVE TRIED PREVIOUS PASSCODES TO SEE IF I HAD CHANGED IT AND I GET LOCKED OUT. HOW CAN I GO ABOUT RESETTING IT ? THERES PICTURES ON THERE I DIDNT BACK UP AT THE TIME OR DIDNT TRANSFER TO MY NEW PHONE AND THEIR FROM WHEN HE WAS A BABY. HELP!!