OP you need legal advice that people here just can't provide.
You need to follow Fuckwittery's advice. If you can't get permission from your husband to permanently remove the children, you'll risk them being returned to him, which will make your situation even more complex.
If he won't agree that they can move back permanently, it might help to view it from the angle that your children are used to Germany, and another big change might not be in their best interests right now?
You also need to secure your bank account, as a priority, and talk to him about where you will reside. If he does give permission, do you have somewhere you can stay here immediately? Do you have the means to fly everyone ASAP - passports, etc?
You'll need to look into whether you'd be entitled to any benefits if you return, and also whether you can satisfy the habitual residence tests required to file for divorce here, etc.
Do remember that any time after you remove your children does not count in terms of them being resident, though. You cannot make them habitually resident here without permission, regardless of moving them here and settling them in new schools, etc. Without DH's permission, it would all be irrelevant should he make a Hague Convention request.