This seems like a kindergarden question to me, But I really have never thought, nor have heard anyone say....

Do fawn does come into heat? If so, will a mature buck breed a fawn? Do fawn bucks breed?

Many of the doe fawns will get bred. Most likely towards the end of the last rut.

Yes. I believe their cycle starts about 30 days later than mature does.

I read a stat about fawns getting bred in MO...don't remember the exact % but it was something like 85% of all doe fawns get bred. They come into heat in Dec and/or Jan here.

Late December in my neck of the woods. These does bring out some true monsters here that you haven't seen all year. Makes late season hunting fun around here.