At the end of the day this is your decision.
IMO a simple one, do you need to sell a horse to cut back, get some money etc then don't trade because you have not done what you set out to do.
I, personally, think the trade sounds a bit too good to be true and would be worried that what I was getting was not exactly what I thought.
I, personally, don't like trading my animals. It just seems second best for my horse to 'swap' them for something else?
I would find out why they are selling this filly and what they are hoping for in return???
Not many 'horse savy' people would trade a registered taller young mare for a young, smaller, unregistered stallion for their kids to ride. I would worry that he may not be what they really want/need and so he may end up getting bounced to another home.
To me, this doesn't sound like the ideal situation for your horse and therefore I would turn it down.
Certainly, if he is gentle enough for leadline as a stallion, it would seem as a gelding he would make a wonderful little leadline guy and if marketed correctly could bring more $$ and a much more suitable home, this would be a win/win situation!!
You seem like a person who thinks as they write and sometimes after they write
(takes one to know one
) but I am sure you have your horses best interests at heart as you came on here looking for advice.
Take it slow and you will make a better decision and one that you are comfortable with, Do you think this is best for your horse??? Figure this out and then decide!
Of course, JMHO