Well, now it seem ideal for the kids. But the kids grow - the pony doesn't. And it cannot do much work, either.
I wouldn't feel good to get it just as a toy for the kids. If I had a place like your's, I might concider perhaps a cold blood like a American Cream Draft Horse :

They have a friendly character, are willing and reliable - and very strong. It can cary all the kids at once, or pull a carriage. And it looks great, doesn't it?