Cyclocross - Bike fit question for a kid

I'm looking to get my 12 yr old son a new bike. He would fit a 52 no problem now but I am thinking of the future and getting something that he could grow into like maybe a 54 or 56. Trouble is the 54's are sold out so next size up would be the 56. He's growing like a weed so I'd hate to get something only to have to replace it next year or so. Does this sound like a logical plan? Appreciate your input. Thanks!

My daughter is the same age and I'd like to have the next bike I get her be one that will fit her until she reaches her full height (more likely with a 12 year old girl than a 12 year old boy, I realize).

Given that with a boy, you're likely to need at least one more replacement before he's through high school (assuming he keeps riding beyond 16), I would be tempted to get the one that fits now and just be prepared to replace the frame in a couple of years. Bikes that are too small can be made to fit with long stems and seat posts. Bikes that are too big are tough to make fit.

This coming from a guy who rides a 52 at age 39....

He's 5'5" now which is pretty tall for a 12yr old and he will continue to grow like a weed. (I'm 6'7"). He's buying this with his own money so it's gotta last for a long time. Decisions, decisions.

I don't know how much more riding weather you'll have in your area, would it be worthwhile to wait until next spring?

Ended up having the Fuji LBS fit him properly. They put him in a 50" and said it would last him a couple years at least. Thanks all!