If the horse was free or almost free, maybe. I would also get my own vet to examine the feet. I would have the ideal dry lot situation and comit myself to a no grass life for the horse and consider the price of hay without grazing to offset the money. I would have a pea gravel dry lot like Pete Ramey likes and bed the stable with shavings. If the rotation was not severe and the horse did not have a huge separation of the white line, I would consider the horse. I would purchase the soft ride boots. I would also test the horse for cushings, which I could afford to run the test if the horse was free. If Cushings was the root cause of the founder, I would have to comit to a life of pergolide at about a dollar a day cost. If this horse had a "to die for" temperament and was truly child safe... maybe, but I would educate my children about founder and what that means for care and precautions. That said... My horse Smokey foundered once, the rotation was not severe, we caught it early. After his feet settled down he went many years sound and it was not complications from the founder that caused me to put him down. I rode frequently and I had full pads put on his feet. I had the farrier out every 5 weeks, no more than 6 to keep up with his trims. Later when I took off his shoes, I just rode in ez boots so the stones would not hurt his feet. He did great. Founder was not a death sentence, but he never was allowed to graze again for the rest of his life. He had a to die for temperament and would have been a perfect horse for a little girl to learn on.
Best wishes... IMO you are taking the biggest risk and if the owner of the horse will not consider a free placement or sale for one dollar to be official, I would probably move on and find something else. The reason I mention free or almost free is because for me to be comfortable taking the horse I would probably spend a couple of hundred dollars checking the horse out first. Founder is a heart break, but doesn't always mean a death sentence or total avoidance, but it does requre a founder savy owner that is willing to make the commitment on proper care. good luck, I know how hard it is to find a kid safe horse you can trust. Free talks. Just a thought.
PS, if the vet I had check out the horse found no rotation, then I would consider a modest fee for the horse. Different vet would be a good idea since I just read your post and realized that you share a vet... best wishes