I don't think you are EVER entitled to hit / slap anyone elses kid.
Actually, I would go one step further. Technically/Legally you are not entitled to hit/slap ANY kid, including your own.
As they are a child, they don't have to be the one to complain about it either. If you are seen striking your child, you can be charged, regardless of your or your child's view of the matter. It is assault, full stop.
Now, I'm a realist, and I think a little bit of corporal punishment helps to re-establishs some life balance from time to time, but watch out for all the do-gooders in the world with nothing better to do than f@#$ up everyone else's life because they haven't got one.... (...*sigh*.... as I dismount my terribly high horse...)