I recommend that you work with your daughter to find someone who is good with money - could be a financial planner but could also just be a friend or relative who is good at stretching a dollar. Have your daughter get all her financial records together and meet together to figure out how much she has coming in on a monthly basis. Then make a budget for all her regular expenses plus savings.
Target the problem areas - which include her working for minimum wage and more importantly, her husband not earning enough to support the family. Look at whether there is another way they can earn more, and just how much more then need to earn to get by.
They need to strip out any extras so they can put something, even if it’s only $10 or $20 a week, into savings in case of unexpected costs like a car repair.
They need a plan to increase their income and the ideas posited earlier of a Pell grant, etc. are good. I was able, as a single mom (with a bachelors degree already - but in a non-marketable field currently) to obtain a government grant simply by talking to my community college, going to a one hour meeting and filling out a one page form, to get my full tuition and books and expenses covered by a government program to become a paralegal. Jobs are tight and mine isn’t perfect, but I went from earning nothing to earning $40,000 a year in one year. Community colleges are a tremendous resource and your daughter should get trained in a skill that pays better - even strong computer skills can get $15 plus an hour in temporary jobs, which often end up as permanent assignments with full benefits.
In the meantime I would suggest you also figure out what you can afford to do to help the family, with no expectation of being paid back, but with a definate termination deadline so they know by X date, they must be financially independent of you. Terminate the emergency bailouts so they are forced to figure that out on their own, but if you can perhaps cover some specific expense for them each month to help out, with the expectation that they continue to share and review their budget with you monthly and show the progress they are making by committing to the plan, I think it’s a reasonable thing for a family member who is able to do. Perhaps on the condition of your daughter or her husband being in school and earning at least Bs, you will pay their electric and gas for two years. Or maybe you will provide the children’s clothing and one park activity for each child per session - whatever works for you. If you cannot afford to help them financially, then that’s a reality you have to share. Your son in law might have to get a second job, such as delivering pizzas in the evenings.
If there is no plan, this will never end. The key is not to make them feel criticized, but rather just that you want the best for them - and you want them to have less struggle than you had yourself.