For most young girls, learning kip (upstart) will take longer than any other skill. Many kids will do good giants before they master kip.
The best kip instructional videos online are posted by JAO.
Here are some others, City of Birmingham Gymnastics with coaches Krystie Newton and Jody Kime.
Click PLAY or watch it on YouTube.
I’d try all of those with all kids. You never know what’s going to help them make it click.
My favourite drill of all is kip on the end of parallel bars, significantly easier than any kip on a single rail.