You've gotten lots of advice... I agree with what Spiveyman says, so won't repeat that. My recommendation:
Locate somebody who truly knows horses. Let them locate a gelding or mare..8 to 11 years old... that is so gentle that it won't untrack when a dog fight breaks out under it.
I mean it...this animal MUST be not spooky, not jerky, not spirited, not nuthing... it MUST let kids crawl all over it, kick it in the sides, pull its mane, and not bite them, rub them off on a fence, kick them, NOTHING.
There are horses out there that fill this bill...you don't care what they look like...just that it is dog gentle. But beware... there are MANY who will tell you that a horse is like this BUT it will have a bad habit that they don't tell you about.
Reason for this...you are inexperienced and your grandkids are certainly inexperienced with horses. Even horse people get injured/hurt regularly and every time it's a "freak" accident... You don' need any more problems than will naturally come your way if you decide to own a horse.
Best deal is to find somebody with a 14 year old girl who has just graduated from a "kid" poney to a performance horse and daddy needs to sell the pokey kid poney. If you can watch the 14 year old with the horse, talk to her... find out if the horse spooks at cars, dogs, people, loud noises, etc..etc..etc.. Test the animal yourself... throw up your hands unexpectedly while 6 feet from the animal... it should do no more than notice that you are there. If it boogers or jumps... pass on it...
So, if you find an animal that will load into and out of a trailer easily, stand calmly when being saddled and when somebody is seeking to mount it, let people crawl under its belly and not get kicked, let people pull its tail and not kick, walk calmly to and from the barn and not suddenly run back to barn, it should not bite or nip people holding the reins...
When you find this animal...buy it... this animal you will love..and the kids will learn a lot from...later, if they still love horses whenthey turn 14, then they can move on to a more performance oriented horse..