Video editing software compatible with OS 10.13.6 High Sierra

Hi, I'm repurposing my old IMac (late 2009 21.5") for a 9 year old to use. OS is 10.13.6 High Sierra. He's really into video-editing, and I tried to download IMovie, but it says the OS is too old (needs 13.5 upwards). Can anyone recommend another basic video editing software that would be compatible? Thanks