I would assume (based on the experience of a friend who did similar) that you would continue to receive your maternity benefits in the UK for the duration of your leave, presumably still paid into your UK bank account which you can then transfer/withdraw here (if you take the unpaid portion, then presumably that would be unpaid). Assuming you're not planning on returning to your UK job, you would then resign and at that point would have to look into claiming unemployment here based on your UK stamps - not sure how long you have to wait if you have chosen to leave a role, I think there's an extra period before you can claim if you've quit rather than been let go. Like I say, this is what my friend did, but I don't know if there were other ways she could have gone about it.
I would have thought claiming children's allowance should be fairly straightforward once your husband is working here. I'd give them a ring and ask, I'm sure they'll be helpful.
Good luck - to my surprise I found it a lot harder figuring out how things worked on moving back to Ireland than I did moving to the UK: it feels much less straightforward to find information like this. My one piece of advice is to try and change your addresses on your bank accounts, etc, before you leave - it is surprising how many banks/utilities don't have numbers that can be easily rang from outside the UK.