I understand you're scared and feeling panicky, but you are engaging in all kinds of "what-ifs" that may not even occur.
You have no idea why he was calling last night--he might have been feeling angry, or guilty for missing the birthdays.
You have no idea whether he will show up today, much less how he will behave or the kids will behave.
Breathing is excellent advice.
I suggest you talk to your lawyer about your options in terms of what kind of limits you can put on contact with the kids. You won't be able to get a domestic violence restraining order unless he has at least harassed you (depending on your State's definition of harassment), but you still might be able to get a civil order that will give you some peace and the kids some safety.
If you can't afford a lawyer, see if you qualify for legal aid or for some other kind of nonprofit services.
You will have a tough time barring him from time with the children without SOME kind of order in place.
Hugs, keep us posted on what happens.