One thing to remember is that ALL livestock has e coli. If they didn't have E coli in their digestive system, they would most likely be dead. I do agree with the saying about the kids on the muckheap. If your children are exposed to the germs, then they will have a good resistance to it. It's when they suddenly met a new germ that they have no resistance to that problems can occur. I am (very) nearly 30, I grew up with goats, the occasional calf, the occasional pig, and nearly always orphan lambs. I have been exposed to most things, have even had Orf myself, and am none the worst for any of it. In fact, I would say that now, I have an excellent immune system, I hardly ever caught a sickness bug, only the occasional cold.
As long they wash their hands before they eat, and make sure any of their visiting friends wash theirs, I wouldn't worry. My neices (aged 12, 10, 7 & 7) all go into the goat pens, the goat field, they play with the kids and they have all been doing that for years. The eldest two have even been present to witness goats kidding. None of them have had ill effects. My mother swears the only virus that had passed between our goats and children, is when my sister gave the goats tuberculous..... Never the other way round.
Beth