It is a neurological condition which will deteriorate with age.
It will significantly affect value and the ability to pass a pre-purchase sales vetting.
As it gets worse, the hind leg action will get more extreme. IME, it doesn't affect what a horse can do - I have seen several high level show jumpers with it, but I would never buy a horse with it.
My best ever horse had stringhalt. I don't know if it got worse with age as I sold him at 12 (or the vetting said 15). It was quite mild though, as he would come out of the stable and do some strange steps and then as he warmed up it went, with just the very occasional odd step. He hunted, evented and was the soundest horse I have ever had.
A horse with stringhalt won the Grand National once, and horses do compete successfully. It does depend on how bad it is though because with some horses it is obvious with every step. I don't think I would buy one like that as it must be uncomfortable to ride as much as anything. Also it depends on how you end up with the horse and how much it is worth to you in other ways.
It is one thing to persist with a horse who develops stringhalt - quite another to buy on. Or - as OP is quite rightly querying - whether this is a good choice to loan for her self and 2 children on lead rein.
Some cases of stringhalt can be managed successfully for years. But eventually it will end the horses ridden career with them becoming increasing difficult to ride and eventually difficult to handle on the ground too. Not because the horse itself is difficult but because the movement of the hind limb is exaggerated and entirely outside the horse's control.
It would not be my first choice for a loan horse where young children are involved I'm afraid.
I have one who apparently had stringhalt, all the classic signs but my very good physio reckoned it was shivers. Not sure of the difference either in presenting symptoms or prognosis but he is very difficult to trim. He improves the more I pick his feet up and is better if I handle the hoof rather than the skin above it, where the nerves are, but trimmer tries to avoid much hoof care for him .