Or, you could do like the half-elf from human/elf breeding and give them normal stats and all racial elf abilities just with the numbers halved.
The normal stats (I.E. no racial modifiers) may sound strange ... but to use the example from a few posts up ...
Hig elf (+2 DEX, -2 CON) and Wood Elf (+2 STR, -2 INT) could very easily get an averaged value of their stats (+1 DEX, -1 CON, +1 STR, -1 INT) and since nobody in their right mind wants a race with +/- 1s in it then you just ignore them all figuring they just averaged out into a statistical "normal" person. So the stats are nothing. any special abilities where both races get +2 to a skill check, the child gets +2. Any racial abilities where only one parent gets a +2 to a skill check, the child gets +1. Any modifiers that provide a mere +1 to something and only one parent as it ... well, these are ignored for the purpose of the child.
That should allow it to be balanced. Granted, its a fair amount of averaging work ... but if you aren't in it for the true crossbreed, then just go with the suggestion above with let the player pick which one it more likely resembles and go with that. In the long run, that answer is the easier one and that answer is the one I'd personally pick.