Inadvertant Whistle and Flag on Kickoff (and cussing)

[foureyesallbad]

K kicks off to R. As R80 runs across his 30 with the ball, my LJ flags R55 for a block in the back at R's 38. In the moment of excitement (he's a HS kid), he blows his whistle. Upon hearing the whistle I immediately bag the 30 and cuss under my breath. :-)

I enforced the block in the back from the end of the run (R's30) as I believed that R lost their opportunity to replay the down because of the flag on them.

Is this right? And was my enforcement correct (10 yards back from R's 30, where the ball became dead)?

Thanks in advance for anyone who can help.
4eyesallbad