My DD is 6 and has been having issues with this for a year. She has great core strength, but lacks upper body strength. Her coach recommended she work on her strength at home, and so now, every day, she does 10 x slow chin-ups, making sure to put her chin on top of the bar each time and hold it there for 10 seconds. 10 x slow leg raises. 2 x rope climbs, hands only. She can now do her chin-up pullover. It's not neat, but it's getting there, and she's now pulling up to it properly, rather than jumping up to the bar and throwing herself over.
Just lots of conditioning, lots of practice, lots of patience - and you'll get it!