If one of my neighbours suddenly showed up offering help I would be a bit concerned that they thought I wasn't doing a good job parenting.
Maybe you can start small by offering to do some shopping for her while you do yours.
It can be dreadful doing the shopping with children and offering to do a chore can take some of the pressure off without nessescarily suggesting that you think she's having a hard time. If someone wanted to take a list and some cash and bring my groceries home I'd be pretty happy to have that chore taken care of.