Edwin, you should be prepared to explain to them how you'll prep your kid to minimize the proposed "damage." That may reassure them.

We did explain a trial grade acceleration carefully to our kid, just in case it really wasn't a good idea. (But it was.) Learning with peers really matters for some kids, and a younger kid can be accepted graciously by older ones. (Sometimes it has a "mascot" quality, which isn't terrible.)

Let us know how it goes...

DeeDee