Question

1) Their children / not read or write/ yet (can)

Answer 1

Their children cannot read or write yet.

Их дети пока не умеют ни читать, ни писать.

Answer 2

If I understand correctly, you're saying that someone's children cannot read or write yet. If that's the case, here are a few possible rephrased statements:

1. Their children haven't learned how to read or write yet.
2. The children are still unable to read or write.
3. The children haven't developed reading or writing skills yet.
4. Their children are not yet literate.
5. The children are at a stage where they cannot read or write.

These variations convey the same meaning, indicating that the children have not yet acquired the ability to read or write.