Family reunification

Hello hotwaxi_art thank you for sharing your question here in the forum.
I will try to give you an answer but I can only give you a general overview today. This is because every individual situation is different and different requirements apply for different people. I hope that it is still helpful.
In general, kids can receive a visa for family reunification with their parents as long as they are minors and not married/divorced/widowed. So it is theoretically possible to get a visa for minors over 16. If the kids are 16 or older, there are sometimes more requirements like knowing German. This makes it more difficult to get a visa. It might apply to your case, but not necessary. There are for example different requirements for parents who live in Germany as refugees or have specific residence permits.
This is all I can say for now because I do not know the whole picture. But yes, you are correct: When the children are over 16 it can be harder to get a visa for them.
I know that it is sometimes difficult to get appointments with the embassy to apply for a visa. On the website of the embassy it says that it can take over one year to get an appointment. Unfortunately, there is no way, that I know of, to speed up the process. The most important thing is that the registration for the appointment is correct - the registration must be for the correct visa and there should be no spelling mistakes in names, birth dates or addresses.
I am sorry that I can not give you better news at the moment.
If you have any further questions please ask them here in the comments. I will try to help you with them.