I've never had children so I can't compare the experiences directly. However, it seems that since a majority (I assume) of people careing for aging parents are themselves parents, that caregiving leads to burnout so easily while parenting children, while at times exasperating, is as a whole a rewarding experience. Or to put it another way, one of the other caregiver web pages states, "[Caregiving] may be the hardest jobs you’ll ever have, and it can take time to adjust and come to terms with it. But try going it alone and you’ll quickly hit bottom." I've never heard any similar statement about raising children. (I've been asked more than once why I wasn't interested in raising a family: the short answer is medical/genetic reasons combined with my own mental-health handicaps.)
The armchair psychologist in me wonders why a majority of adults have loved bring up their kids - "terrible two's," teenage years, school issues and all - yet caring for a senior or disabled adult is almost universally taxing to the point of being hazardous. In summary, what differences have you seen between your child(ren) and your adult-in-need?