Wow! I never cease to be amazed that anyone would be foolish enough to use RoundUp, but for a mother to have her kids out spraying it is unconscionable. First of all, I would not be going into her yard to help her pull weeds. Roundup has been found to have a very long half life, up to 141 days! You are putting yourself at risk by going into her yard. It gets on your shoes, clothing and skin and then you carry it into your house. If I were you, I would also not let your children play in her yard.
I would be very up front with this woman and completely direct. I would tell her that Roundup is toxic and that neither you nor your children will be visiting her property. My guess is that she also sprays other lawn chemicals and pesticides in her yard as well. I would direct her to the internet to read articles such as this one: http://www.mindfully.org/Pesticide/Roundup-Glyphosate-Factsheet-Cox.htm
If she puts Roundup above your friendship and neighborly congeniality, I simply would not associate with her. If you are directly downwind of her property or any run-off from rain comes into your yard, she is putting your health as well as that of your children and pets at risk. I think that is a most un-neighborly thing to do.
In the neighborhood I live in, at least 80% of the people spray their yards with toxic lawn chemicals. I constantly see people out spraying Roundup all over their sidewalks and then a few minutes later little kids are walking through it, parents are pushing baby carriages through it, and joggers are running over it. Unbelievable. I have no idea why this stuff is legal.