What are some ways to help children, especially young ones, who are not doing well with online learning at home?

Prior to giving any technical support, I would first like to ensure such "troubled" children that it is not at all the end of the world and that they have ample opportunities even alternatives to survive, overcome difficulty or lead enjoyable life. I would also encourage them to hang in there no matter how slowly or late they learn rather than quit altogether. It's sad to see many promising young tend to become too pessimistic or even suicidal, often about small things.

I recommend for parents who home school to watch Sir Ken Robinson on Youtube. He can help people become better teachers

I have a cousin who was homeschooled and what I got from my aunt was to make learning fun while paying more attention to the actual process than just force feeding the kid to memorize and learn a lot of stuff then have them regurgitate everything perfectly. For example, when teaching history, teach it like you're telling a story or give them actual problems they can grasp in math. Giving treats when the kids get good does help.

Jump in and help them. Seems obvious to me.

Read aloud to the child from a storybook, while snuggling together on the sofa. Difficult to do when the parent is engrossed in their stupidphone.

I think it's the responsibility for parents and teachers to make learning enjoyable. If children are having a difficult time studying, it's certainly not their fault. To make online teaching more interesting, teachers can show them videos and have young ones play educational games, instead of giving boring lessons where they ink take notes and memorize what is on there. This boils down to the significant flaw of Japanese education system, where rote memorization undermines students' creativity and problem-solving skills. I'm certainly the victim. Reform is necessary.

I recommend for parents who home school to watch Sir Ken Robinson on Youtube. He can help people become better teachers

Hear, hear. RIP, Ken.