This is why it's so important for children to be taught internet safety and to be supervised until they've grown up enough to be responsible. If a minor is so unwell that they can't keep themselves away from adult-centered external communities, that's a good sign that they're too unwell to be managing their own internet usage.

Internet safety can be challenging! Plenty of people, minors and adults, use the internet to self-harm. However, adults are responsible for themselves and minors who can't be responsible for themselves need to have that responsibility taken up by their parents.