When mine were young they grew up knowing that their mum had to be careful a lot of her time, because she had a bad back. What a bad back was and what was involved was not explained to them until they were older.
I would keep it as simple as possible on bad days by just saying Mummy hurts, and just needs to rest. My daughter, being typically female would be the one who would go that extra mile when she was very young, by pretending to make me a cup of tea from her own cups and saucers. Her brother (three years older) would keep an eye on her to check her help was not really a hindrance to me.
Over the years they have both contributed their care for me in their own sweet way. My husband used to travel often and could be away for a number of days, but they never gave me grief or upset.
I always had to reassure my daughter more, as she seemed to be the most anxious when I went into hospital for my operations but I think my son always like to play the older brother act.
Letting them do the odd jobs and then rewarding them made them feel very special and they have both turned out to be very caring and understanding children that I am very proud of.
I would say to any young mum or dad who maybe suffering from chronic pain that you should talk to your children about it, in the best way you can, without frightening them.
As they get older let them help you as much as they can without putting any pressure on them. Im sure it made both my children grow up into strong, caring and helpful individuals.