He does not have any legal right to take them overseas without your permission, and I would refuse to give it.
(I’ve travelled quite a bit around Africa in 8 different countries and I always had an address even if it was just a campsite.)
If he isn’t interested enough in the children to look after them over night, why does he suddenly want to take them to Africa for a month? My worry would be that its for things like female genital mutilation and forced marriage, which in some cultures in Africa the dad genuinely believes these are the right thing to do. Every year British children are taken out of Britain ‘on holiday’ to find that these things happen to them.
If they are at risk of being taken without your consent, you can ask the government to put a stop on their passports so they can’t be taken out of the country.
It’s awkward that you have already agreed to a trip. However, you did not agree to a term time trip. You might wish to point out that taking the children away during term time is a criminal offence in Britain, both for him AND for you, he’s basically forcing you to commit a criminal offence for which you could both be fined quite a lot of money.
Reclaim your authority. Tell him that this is not happening because it’s a crime and that he must cancel the flights. Tell him thet you will report him to the police. He will be furious, obviously. A compromise might be that the children only go during school holidays and that he pays for you to go too and you stay with them all the time.
But I would not allow it: even a tiny risk that they never come back is far too high. Children disappear into slave trafficking every day in Africa. How much do you trust him to protect them from that, for a month? I don’t trust anyone that much.