My feeling is you need a non-kid-climb barrier and something people can’t easily toss things through.
And you need it NOW.
It also sounds like your stock can reach through or over the fence to sample what the folks offer and demolish whatever you choose to plant.
I would lean toward reinforcing the existing fence and since I am cheap I would probably look at using very long rebar as posts 15’ sunk 4-5 ft in the ground. Rebar should be tied to the existing fence. I would buy 1”, 6’ tall chicken wire (poultry mesh) 150’ lengths and run a light cable through the top and also through the bottom. I would hang that at the top of my existing fence on the rebar (6’ above the current height. Reinforce the corners with ‘tent-post- type cabling on the rebar. Rebar spacing as needed, probably 16’ or more.
So your reinforced fence is 9-10’ in height. They are far too tall for the horses to impale themselves on, the chicken wire discourages climbing and reaching through and lobbing doesn’t work as well as pitching at the horses.
I don’t know what your existing fence is, but mesh or rail are both easy to climb and throw things through. So I’d likely net the bottom, too and tie that to the top chicken wire panels.
The chicken wire should last 3-5 years, long enough to get a hedge going. I suggest tall pricky junipers, but they are slow. Honey locust with thorns
is safe for stock to eat and very discouraging to kids.
You can coppice fast-growing trees like willow and poplar –check the state forest service for suppliers.
Then I would put up signs near the corners GATE(with an arrow) and at the GATE: GUESTS PLEASE SIGN IN AT OFFICE maybe with your phone #.
People throwing things and chasing the horses are not guests. They know that.
Hot wire is your friend. Good luck.
Oh, I have yet to find plants that deer
will not eat - tomatoes, cherry, walnut, apple, oak, maple, EVERYTHING, 'poisonous' or not. I trust livestock less than the deer and it is my vet bill if they get sick, so we are trying to fence trees and shrubs off within the pastures, no matter what they are. And due to the deer our yard is a series of cages round the desired plants.