I'm a SAHM and split with ex over 2 years ago. As my DCs are all school age I don't get Income Support but I would think you would - that seemed the most generous benefit and also opens doors for other freebies (free school meals/uniform, council tax reductions etc). It will all change to Universal Credit at some stage but I don't suppose that will be a quick thing, so you have a couple of years I think.
As the mum of a pre-schooler you won't be expected to find full time work either, you can remain a SAHM or work part time if you want to and you will get financial top-ups from tax credits for childcare and other expenses.
As sad as it is, I've ended up in a pretty good situation as a lone parent - I worried that I would be disadvantaged, but actually my ex was very financially controlling and so I've benefitted from being on my own and being able to control my own money. He pays the CSA amount for the DCs + some extra to me and I've remained in the family home - as a mortgaged property I don't get any assistance with that, but if you rent you will get housing benefit too.
CAB were pretty useless when I went in to see them in the early days, I sat in tears and they basically showed me how to use their website, which I could have done at home on my own.
However, MN has been a great source of support for me and once your confidence has increased a bit you will be able to sort out paperwork and claims pretty easily yourself. There's always this panic that if you get it wrong you'll be in huge trouble, but as long as your finances are pretty simple and you have all the info you need to hand you'll be fine. Good luck and keep talking to your STBXH as that is the key to sorting it out amicably.