It sounds like you really found something that brings you a great deal of happiness and good bonding with your daughters. I'm absolutely thrilled for you. Being around horses is an amazing thing. They are such great animals and can really heal people.
Just be cautious, the last video was a little hard to watch, he's a 'hot' horse so being excited around him is going to amp him up, which isn't always a good thing. They may act like dogs sometimes but they are still huge animals, as cute as he seems to be acting, its not a good idea to amp him up like that. Its easy to forget that their behavior if you think they are being cute isn't always good behavior. You spoke to him like you would to a hyper happy puppy. Which if he was, would be perfectly fine. You could talk to a horse like that if its a horse you really know and know is gentle and you have a bond with, but getting horses excited like that in a stall is just never a good idea, especially if he has the problems you previously described. Even with my horse I'm always quiet and calm, you never know when something is going to set a horse off. He's in a small space and he's a 'hot' breed so if anything try be calmer around him, speak softly, love softly.
I'm not trying to put you down at all, I love the fact that you have found a new love. Especially one that you can do with your daughters, I wish my father would have been more involved, it would have been an amazing bond to share with him.
Quiet and calm is the approach you should always take with a horse, be confident and be the leader. Treat him like the huge animal he is. I think you're in a great place and doing good work with volunteering just don't be over confident, that's the easiest and quickest way to get yourself or the horse injured.
I think everyone here wants you to be successful in this but are just worried about how you're going about it, and sometimes they can be ugly about how they try to give advice. It is a little scary how you're acting around this guy, and horse people can get very up in arms about the animals. I'm really not trying to put you down or anything, just caution you.
We need more volunteers with a heart like yours! Especially with working with OTT horses, volunteers are critical, especially ones who love it, and you clearly have the bug! Horses are amazing animals and its nothing to be ashamed of as the only older guy there, you're in a great position to learn lots and help out a lot more than the little girls, who certainly help, but sometimes strength and being older and wiser is a lot more welcome.
Keep learning, loving and feel free to ask any questions.