I would also use the fact that he's being 'reasonable' to sort the finances asap, before the guilt wears off, or things become more strained.
Its the last thing you need to deal with now, with all the emotional turmoil, but as others say, if it turns out there is someone else involved you will rightly want to scream and shout, call him every name under the sun. If you are worried about how this will impact on him being 'reasonable' you won't feel free to react in the way you naturally want/need to. Even without another woman, you will be feeling up and down for a long time yet and you don't want to have to watch what you say to him for fear of upsetting him.
You will get help with your income if you're not earning much (this isn't affected by how much maintenance he gives you, so make sure you get a good deal from him). You can also get your council tax reduced (maybe even to £0), you will still get child benefit and you can get cheaper deals on your water bills etc when you are a single parent on a low income.
Your DCs are the same ages as mine, so you won't get Income Support, you will have to work a minimum of 16 hours a week (can be term time only) and you probably won't get free school meals or help with school trips etc. but you will find that you aren't that much worse off than before if your H can give you a decent amount to help with the mortgage.
You will also have the incentive to try and earn more, either by expanding on what you're doing now, or looking for something more stable (tricky working around the children I know). You can get help with the cost of childcare too, so don't let the hours put you off if you find something a bit more full-time.
Hope you can get something sorted - mine has been nearly 2 months and I'm still finding my feet, tax credits are still not through (they are really busy and it may take up to 8 weeks! But it should be back-dated).