What happens if these parents die suddenly, in a car crash for example? If there is no will in place, a court will decide who cares for the children and who manages any assets left to the children. This is a terrible situation for the children for many reasons.
First, the kids will be in limbo, with a temporary guardian, until a final decision is made about guardianship. It can take months and months for a decision to be made. It is a tough decision for a court. Local friends often become like family and can be the best choice for guardian, but courst give a lot of weight to family relationships. Different interested parties may chime in and prolong the issue. And, not to be cynical, but relatives tend to appear when inherited money is involved! So, your kids may be here and there while all this is sorted out- not a great situation for a bunch of kids who have just lost their parents!
Second, and this is crucial, once you are gone you have no say over who cares for your kids. As stated before, local friends can be much better guardians that relatives, but the court may give more weight to the blood relationship. Your kids may end up with a second cousin in Nebraska that you met twice. Also, you may want your kids to be back in your home country. For a court to take this step, particularly sending US citizen kids abroad, you will absolutely need it spelled out in a will! (If you want your kids back in your home country, a well-drafted will should set-up a temporary guardian to watch your kids until the end of the school year or until other logistics can be worked out).
Third, the money you have saved for your kids is in danger of being lost. Many non-immigrant and immigrant families save and save, for tomorrow, next year, and the next decade. This is hard earned money involving years of college and moving to the other side of the globe! If you die and leave the money to your kids, the first problem is that your kids can gain control of the money by age 18! This is money that was put away for college but may be wasted by a kid barely out of high school. The second problem is that the court decides who will manage the money. This is where you find all kinds of creepy distant relatives showing up with toys for your kids. In your will, you name who manages the money and can pick someone trustworthy.
The hard part about doing your will is making these decisions. Do you want you kids here or in your home country if you die? What is most important, being cared for by blood relatives or staying the US uninterupted? And who do you trust with your money? Yes, these are hard decisions, but they have to be made. The immigrants I've met tend to be incredibly responsible, especially when it comes to kids. Doing your will is part of that responsibility.