Hi backflows,
Being a cattleman, I recognize that you might be headed for a problem with a cattle herd grazing, even temporarily on your land.
Safety for you and especially your children is the main concern here. Without knowing the breed your neighbor raises, I would point that cattle are very large, not overly endowed with brains, and not necessaily docile, especially when cows have calves on them or a bull is with the herd.
Also, cattle browse all plants they can reach and my Charolais can reach a good 7 feet in the air, so if you have small trees to protect, keep this in mind. Also, cows and bulls will scratch against small trees and lean against any structure like fences and unprotected outbuildings. Cattle are not discriminating about fouling water either, so keep your water source in mind.
A solution may exist - consider asking your neighbor if he would mind putting up a solar powered moveable electric fence to control where the cattle graze. They learn quickly where the fence is, especially with some red tape tied every 20 feet at first. And the fence is safe for humans, though it will wake you up if you touch at the wrong time.

Finally I will say that all this is not to discourage you from helping your neighbor. You get your pasture mowed for free and cattle are great to watch and a whole new educational experience for your kids and guests (from a distance!) I have never met a cattleman who was not a complete gentlemen and very considerate, and I'll bet you will have found a great friend in the area if all this works out.

Cattle and trees do not mix!

Any Pressure from cattle walk around trees will cause root problems and eventually, over some period of
time kill your trees. Also, they will chew on the bark.
Kornad