There are some dodgy opinions on this thread. I think the fact that he is going to West Africa is bringing up some crazy.
That said, the following would concern me:
He never has them overnight.
A month is way too long, given that he never has them overnight.
Missing two and a half weeks of school. That will also concern the school.
Fines for missing school.
You were married for 14 years, right? And you never went there, don't know any of his family? You being totally unaware of where your dc are for a month, in a foreign country, with people you have never met... that would worry me.
I would say they should go for a week, this first trip, and two weeks at an absolute maximum. His idea that a week isn't worth it - that's an adult perspective and a symptom of that selfishness you mentioned. In a week, they can meet their cousins and grandparents and aunties and uncles. See where Dad is from. Get to know everyone.
I think you should go for this trip. Meet everyone. Shake hands, have tea, get to know your dc's family. Then, after a week or two at most, you can fly back with them to the UK, if your ex wants to stay longer.
And next time Dad is able to plan a trip, you will be much more comfortable about it (or alternately, know never to send the dc there again!).
By the way, you being there does NOT minimise the risk of him abducting your dc and keeping them in the country of his origin. But if you feel that is very unlikely anyway, then go ahead with a negotiated version of the trip.