Hi @Mjensen
She cannot renew her expired residence permit. If she wishes to move back to Germany with at least one of her German children, then she must apply for a visa for family reunification with a German child at the embassy in Bangkok. Here is the information on this from the German embassy about this:
https://bangkok.diplo.de/th-de/service/05-visa-und-einreise/visum-zur-familienzusammenfuehrung-1603984
You’ll notice form the name of the visa and the information provided by the embassy that such a visa is most commonly issued when the German child already resides in Germany and the parent wishes to join them. However, it is also possible for this visa to be issued on a discretionary basis when the child is only now moving to Germany. For this, she will have to make clear that the child is ending their residency in Thailand and are moving back to be in Germany on a permanent basis.
If the non-German children are also planning to move to Germany, then they also may require visas. If one child has Danish citizenship and passport, they do not require a visa. The other child only has Thai nationality and so would require a visa. This would also be a visa for family reunification but in this case based on the link to the mother with some other requirements.
The person can also write any questions she has here in German or English.
Best,
Éanna