I'm building a balance bike for a daycare kid. He is tall so I have a longer frame for him, but I think the tires might he too big. Can I put 12.5" tires on forks for 16" tires?
Assuming: - the axles are of a width that they will fit the dropouts and can be tightened down enough to prevent the wheels from falling off - You don't have brakes or a drive train to consider (which you said you didn't in the comments)
...then why not? The bike will look a little funny, but the kid will dig it.