Hi @HK97
In my view, your question has two aspects, namely immigration law and family law. We are not lawyers here but I can give you an initial assessment of the situation based on what you have written.
Immigration matters:
As I understand it, your child was issued with a visa to come to Germany under family reunification and was then here issued with a residence permit. As long as the child fulfils the requirements for this residence permit, then they can continue to extend it.
An example of no longer fulfilling the requirements is if the child turns 18 years of age. They then are no longer eligible for this type of residence permit and so would have to apply for another type of permit.
I do not see what influence you can have regarding the validity of the child’s residence permit at this point. While you can formally retract your consent to the child moving to Germany, the child is now living in Germany and presumably their main place of residence is in Germany.
Family matters:
Part of custody rights in Germany is the right to decide where the child lives. If there is a dispute between parents where the child should live, then a family court can make a decision on this. The best interests of the child will always be the main deciding factor in this.
If you have concerns about your child’s welfare and maintaining contact with you, you can get assistance with this. The authority responsible for ensuring you have access to your child is the Jugendamt in Germany or an equivalent organisation in your country of residence.
The relevant legal basis of the access rights regarding a child are the laws of the country the child is living in, i.e. in this case Germany.
The clearest case of how a visa could be cancelled is if the parent in Germany no longer had custody of the child. Presumably the child would then initially be taken into care and then returned to the other parent, if that parent had custody. But that would be a rather extreme situation that would involve the family court and a lengthy process.
I recommend you make contact with the organisation ZANK first of all. They have expertise in dealing with cases of conflict across borders around issues of access and custody rights. They will know far more about this than me:
https://zank.de/en/
As mentioned above, you can also make contact with the Jugendamt where you child is living and request their assistance.
Best,
Éanna