Your "next-of-kin" is your husband, according to the law. then your children. Mom and Dad come after them now that you are married with kids.
I didn't catch the loan bit until late in your letter, but i would make every effort to pay the loan off as originally planned, and simultaneously work out what you and your husband have for goals for family life (you guys and kids) and financial.
You will need to draw clear boundaries, and you will have to tell your parents a very firm, "no", and stick to it. At the same time, you have to stop buying and selling between the families. Become two distinct families.
You may, however, if you have the energy, invite them over to dinner for social time. That might help take the sting out of the situation, by saying, "we want to be family, but we don't want to be used." Of course, family dinners may end up being times when they try to manipulate you guys into doing their work for them again.
the best thing you can do for your children is to bring them up in a loving home, with healthy family relationships all around. Being walked upon is not healthy, nor is staying up 24/7 to do your own chores and/or your parents. Your children will grow up copying your behavior, so if you don't want them to be manipulated as adults, you have to stop being manipulated.
You may lose your relationship with your parents, but it doesn't sound like much of a two-way street to me. It sounds like one heck of a burden on you, and it's time to stand on your won four feet (yours and his) and be independent. That means that you don't ask them for favors, either. and if the buildings are too expensive to own, sell them. Same goes for dad. If he can't afford to keep the building in repair, then he should sell it, not look for free labor to keep his commercial enterprise going.
We would all like to see our familial relationships as generous and we would all like to have the strength to be there for our parents on a regular basis. But they are way over-stepping the bounds of propriety here, and you need to simply say no. We can't. We have other plans for the evening.
And make plans for family time. Take the kids to the zoo. Go to the beach (when summer comes), etc. You need time to relax and enjoy each other as family. In 18 years, which sounds like forever away, your children will be up and grown, and you will have to bite your tongue rather than step on their toes. So the time to enjoy them, and to do things with them is now. Don't let mom and dad keep you so exhausted that you have very little to give to each other and your children.
Be firm. Be fair. Be consistent. Hold the line you draw in the sand, and simply say no. Avoid any shouting matches. Just be firm. Treat Mom as if she were a 2 yr old. Don't argue, don't give reasons or excuses. Just say, "we can't do that. We are too busy right now." And stick to it. It'll hurt. It'll be hard. But when they have stopped looking to you guys as free labor, then maybe you can get back to enjoying being a family that includes them as well. For now, be sure to have family time with your husband and kids. They are your most important human relationships.
:-)