Yes. It is legal for your children to be here. If they hadn't been taking turns throwing temper tantrums for the past hour and a half I wouldn't care.
There are two of you there with them as parents. When your kid starts screaming, take the damn kid outside until he calms down. If they both start screaming at the same time, that means it's time to pay your bill, get your food boxed up and leave.
Try socializing your kids another night when maybe they aren't so tired. If they do the same thing next time, teach them to behave at a Mcdonalds.
Yes, you have a right to a nice evening out as a family, but your right to a nice evening out is not more important than the rights of everybody else in the bar to enjoy their evening out.
The parent of four kids, sitting there with two of her quite children that know how to behave in public and were taken outside of restaurants until they learned how to behave.