Just been through this with my son (not yet 7 1/2, 138cm, clumsy as **** ), and decided on the Beinn 26, arriving today hopefully, after matt had mentioned on another thread what he'd done.
Hadn't entered my head that the fork could be replaced with suspension when/if necessary, but it tipped the balance- no need for disks at my lads weight - especially Avids, which have been problematic for me. He would also have been right at the lower height limit for the Creig.
Less maintenance, less weight, more stand over, less cost.
Now, have to deal with my daughter's request that hers be powder coated into nice pale lilac. Cheaper than a new bike though.