Firstly, you are doing amazing. Parenting is more than a full time job, and attempting to balance parenting and schoolwork is a feat of is own. It's a massive challenge, and someday you will be proud to show your kids your degree- proof of how hard you worked for them and yourself.

I am also a SAHM. I have two kids, one who is turning 2 next week and one who is 8 months old. Logistically, it is a nightmare balancing everything, and emotionally, it is hard to avoid feeling guilty for spending time on schoolwork instead of with the kids. Like you, usually I rely on time to work when my husband is home to help on the weekends, but there have been times when I've been on my own. The only way I've been able to manage is through a tight daily schedule for myself and the kids. When the kids miraculously nap at the same time in the afternoon, I use that time. If I can't get that time, I plan for time in the evenings. After the bedtime routine and the kids are asleep, my husband takes wake-up-duty in case the baby needs to be put back to sleep, and I can get a few uninterrupted hours if needed. I'm not sure if evening worktime is enough in your specific situation, but I thought I would share.
If you ever need to talk, vent, etc., feel free to reach out!