The Ann Summers store in our town is split into two sections, the Knickerbox everyday stuff and then futher back the sex stuff bit.
There shouldn't be a problem taking a child of any age into the everyday section, by not doing so gives that child a negative message about women's underwear etc. It's not a dirty subject that should be hidden. However if the mum was shopping for the sex stuff then she should really save it for when she doesn't have the kid with her rather than leave the poor thing outside.