No, the party at fault is the party who strikes the child.
The child was crossing the street while under supervision of an adult.
The government employee struck the child.
Well, gee, then. That right there justifies walking into traffic. Come on kids. Follow me. Let's walk across this street. Be sure to step out from between some parked cars. Yeah, there's cars coming, they'll stop. They have to. It's the law. Physics? What's that? Gym class?
I’m going to take a different tack with you: why are you worried about fault? Let’s just assume that the off-duty cop is at fault. Where are you going with it? Please don’t tell me that the operative difference is between governmental, and non-governmental employees “at fault.”