I've not come across a mixed tri that allows parents and kids to race together - and frankly if I did, I would refuse to officiate at it!
parents are bad enough shouting at their kids form the sidelines - I have seen kids in tears because of the way some parents react - so I would hate to think what a controlling parent would be like if racing alongside their little dahlins.
in fact, parents should be banned from being anywhere close to their kids in a tri - the kids are resilient enough to get on with it themselves with a little help and encouragement from marshalls and officials who have no "gene investment" in the kids
that's perhaps an over-reaction on my part as most parents are fine, but the small minority of controlling ones don't help the overall ambience of a kids tri and just make it unpleasant to be at.