What can Japan do to prevent child abuse?

Nothing short of a huge social change, that is what it takes to solve problems so ingrained in society. Beginning with erasing the negative prejudice against coming out as a victim and ending with an effective and practical system to deal with even the suspicion of abuse.

Oh where to start...

How about actually vetting and training people properly to be competent welfare and social workers who take initiative and actually conduct thorough investigations the second a red flag on a family is raised?

The implementation of joint custody and standardising child support payments is another so the child can actually be with the parent who wants them; not with the parent who has spirited them away just to spite their ex.

Training teachers to spot and report signs of abuse and training kids to speak up and also report to a trusted relevant authority figure if they are being abused.

Some people just cannot deal with the pressure of being a parent, because lots of Japanese,never experienced emotional support as children

Indeed.

but that doesn't solve the problem.

I already wrote it - it can only be a strict law and a very effective system of detecting family abuse\neglect and taking children into specialized centers. Not only grownup children but especially infants, toddlers with whom welfare center workers seem to reaaally really dont want to deal due to obvious reasons. However, if there's a strict law and authority body to keep centers and parents in strict control and police doing their job properly then everything will be just fine.

Some people just cannot deal with the pressure of being a parent, because lots of Japanese,never experienced emotional support as children

Japan has already found the ultimate solution. Stop having children. No children means no abuse.

Start teaching children in pre-school about the offences, dump their uniforms, teach elementary school kids that such behaviour by their parents in any form means those parents are criminals. Won't take long.

Encourage adoption by stable families, and allow divorced fathers to meet and protect their children from mom’s new boyfriend.

Mandate shorter work days, so that parents aren't too tired to raise their kids with less stress.

draconian rules for such offenses. Japan loves to go draconian over petty offenses, why can't it go ham fisted over things that really deserve a heavy handed approach.

Mandatory death sentence for child abusers.