German citizenship is passed on strictly by birth - the 2000 birth year cut-off is a relatively recent law to prevent the endless passing on of citizenship by people who have no meaningful ties to Germany. If your dc are teenagers now, the 12-month deadline for post-2000 births relates to your children's offspring, not your own. Assuming your dh was born pre-2000, and was a German citizen at the time of your children's birth, your dc would automatically have Gm citizenship as well, though you may need to jump through some paperwork hoops to get it made official. If your dh has an expired passport, or his own birth certificate showing his German parent, and possibly his German parent's birth certificate as well, it should be straightforward, although the process is quite paperwork-heavy.
If you can locate plausible relevant documentation to show their entitlement to Gm citizenship, the first stage may be to make a name declaration, depending on whether or not your children's names comply with German naming law. https://uk.diplo.de/uk-en/02/naming-law/name-declaration-child/2454526 This link explains the relevant rules, and also lists the documents you require. Once you ahve the name declaration, the passport is pretty straightforward.
You are a bit late to the party in some ways, but that may work in your favour - in the year or two after the Brexit vote the German embassy was overrun with Anglo-German families frantically getting their paperwork legalised, and trying to get an embassy appointment was akin to getting Glastonbury tickets - lots of pressing refresh on the website. That will hopefully have calmed down now - we've done passport and ID card renewals more recently and been able to book appointments on line within a reasonable timeframe.
Read the list of documents very carefully, and bring all everything it tells you to, plus anything else potentially relevant - iirc you need triplicate copies of everything. Iirc both parents and the child have to show up to each appointment - I can't remember whether you could make one appointment for multiple children, but possibly not. Also iirc the process is slightly different depending on whether your child is u14, 14-18 or is an adult. Read the small print carefully. The process is cumbersome and nitpicky, but the prize is worth it.