It was easier for us when DS was smaller because we kept him in his stroller and he stayed with our group the whole time.
There were a few years we did NOT take him because our walk is held at a park and he wanted to play with all the other kids on the equipment. Another reason is that DH and I are the Walk Chairs, so we're busy helping set up, speaking... and my MIL just isn't as vigilant as we are. They have a LOT of food -- donuts, cookies, dessert breads, juice, soda, pizza, granola bars... and I try to avoid the food that is open and EVERYONE is touching and I use lots of hand sanitizer.
This past year, we told DS ahead of time that we didn't want him on the playground equipment and that we would take him to a park near his grandparents house later in the afternoon.
Being that Jessica is older, I imagine she just needs to keep her distance -- some places have stickers identifying who has cf. And like I said the other thing is the food issue. If she's going to partake, see how it's handled and maybe opt for individually packaged items.