@elliejjtiny To get back to the original question, I think when funds are tight (for whatever reason) there has to be compromise.
Without knowing the detailed ins and outs of your finances, your situation sounds unsustainable, given that dc at uni may be an increased cost commitment for you, at the same time as there will be a reduction in income due to certain benefits ceasing.
So you ought to temporarily throw out all of your current assumptions, and examine every aspect of your life to see whether there are things that you are doing that are resource intensive (either financially or physically), that you can do in a different way.
Your dh being away for 15 weekends a year sounds difficult - that is nearly a third of the weekends in a year? Does that mean he is then around for some days in the week on a regular basis? If so, can you then work during those times? Can he try and move towards earning more?
Can you move your younger dc towards a bit more independence, re: school runs, as that might enable you to have greater availability of work?
Unfortunately, where funds are limited, cloth does need to be cut slightly accordingly (that is true for everyone) - and it sounds like that is something that does need to be considered. If you do not have the disposable income to support your older dc at uni (once their benefits drop away), then this needs to be addressed. Unis with expensive accommodation should be off the table - unis with cheaper accommodation should be considered. Closer unis that do not require such resources to get to (in travel / hotel / time costs) should be considered. Or taking a gap year or two to work to build up funds to manage. Or staying at home for uni, and going to the local one, and picking up part-time work locally. Obviously this is all fixed for dc1, but not too late to make changes for dc2.
Also, one thing I have noticed on the HE boards here, is that there is an assumption that dc at uni need a large amount of spending money for socialising etc (sometimes £500 per month), which we do not do with ours (£300 per month, which includes all food (in self-catered), bills, mobile phone, and socialising, and travel, and course equipment costs).