Read what you wrote.
There is a huge fantasy life that he thinks he will get with her as she shares the same hobby and they have dreams of travelling the world together having exciting adventures that I can't/won't do with him (way too difficult for me).
Where's the concern for children there? He doesn't picture them in his fantasy life, he wants to go travelling with her, not you, not your children. He wants to abandon you all so he can shag her across the globe. When it all goes wrong and he's hit with the reality, he's going to want to come crawling back to you, if you accept him then he learns he can do it again.
Even after he left we still got on really well together (weird I know) and so this past four weeks with him keep changing his mind has been particularly hard.
He's being nice because he isn't sure how badly he will come across during a divorce or how much he stands to lose.
One of my friends told me at the weekend that to put myself in a vulnerable position is actually a brave thing to do and I think that she is right. Me telling him to sling his hook doesn't actually achieve anything at this point.
Your friend is talking absolute crap. She's telling you what she thinks you want to hear! From a completely objective outside point of view, you're voluntarily walking towards a breakdown.
You're happy for him to go shag someone else and then come back to you. If you take him back he will do it again and again and ping pong between you and whichever woman currently takes his fancy.
He doesn't care about you or the children as much as himself. He is dithering over choosing you or another woman, if he doesn't know he wants and loves her, then why was she worth destroying you for? If he does know he wants her then he'd biding his time to make sure he gets the best possible divorce settlement, possibly being nice to you will continue until the divorce is final so he can have a good shot at screwing you over.
Please get rid of him. Do not sleep with him. Think of your children, he's putting them through Hell and he knows it.