Just my opinion of course, but you would have done much better to leave the sapling to grow to maturity
for your kids kids. (or even tend it for 30-40 years for yourself!)
I don't s'pose there will be any heartwood in a sapling that size.
If you want a small diameter bit then take a branch from an older tree.
I just cut a bit from a local churchyard (with permission and a donation to the church) I can touch finger to thumb round the branch at it's centre, and it's nearly all heartwood with a thin ring of sapwood around it.
I hope you are not in the UK, as we need all our Yew saplings
Del
