I understand your frustration. I have fibromyalgia and it takes so much out of me. I am guessing your husband is responding to you out of frustration. I imagine he feels terrible that he doesn't have the energy or is hurting so badly he can't do things with his family, I do at times. My biggest problem is that I will then push myself and pay severely for several days or a week :(
As far as explaining to the kids that Daddy is too tired or hurting too much and can't come and play, say exactly that. His chronic fatigue and pain will likely be a lifelong condition, mine is, so they need to be aware of it from an early age. Tell them it doesn't mean he doesn't love them, he wants to be with them but his body won't allow him to. It will hopefully make them more compassionate human beings.
Having chronic pain I commend you, I know you have a full plate. Wait until there's nothing planned and ask your husband if there's anything you can do to help him, without putting any pressure on him. If he's in physical therapy or counseling or support groups encourage him to go, and consider going for support yourself. I have a friend who has fibro as do 5 of her children (the 6th is adopted.) She hurts terribly as do her children, but she is a happy camper, she truly inspires me. I guess we just have to stay positive and cheerful, even through the rough times.
BTW, my soon-to-be ex-husband has always said I was lazy, and even after I was diagnosed said i could just "shake it off." He went so far as to throw me out of the house 7 years ago because I " didn't help him" by going back to work, I couldn't! I'm not the type to wish anything bad on anyone, but I think if he could live in my body a couple of days he'd regret everything he ever said to me. So I know how important love and encouragement are.
Just continue to love him and be there for him as I hope he is for you.
Hang in there and God bless♥