As I've said, I've traveled alone with them before. If I need to get up, I ring the flight attendant and they are happy to hold my youngest. It is part of their jobs, and I have never had one ever be unwilling or unhappy to help me out. Often I'll have other passengers offering to help me as we'll. (obviously I don't ask when they are busy)
Actually caring for your child even for a few minutes is not part of their job.