Do the children bear any sin from their mother uncovering a large part of her body in the home?

A woman must repent to Allah and cease revealing what is unlawful of her body in front of her sons and mahrams. Her sons must advise and guide her with gentleness and respect, urging her to stop this act which is forbidden by Sharia. If she insists, then it becomes unlawful for them to look at what is forbidden to them, and they must turn away from her as long as she persists in that state.