They should relax. While it would be appropriate to take your child out of meeting if the child is screaming or being noisy for long periods, the occasional noises of small children are generally welcomed. Some paper and crayons, or a book to read for older children is often helpful, too. Most children, especially those of visitors, have a tough time sitting silently for a full hour. If you see an adult rising after the start of Meeting and all the children filing out, they're probably headed for the First Day School.
Friends are generally quite tolerant of babies and their noises.
It should be considered normal at most unprogrammed meetings to breast feed babies during meeting.