its disturbing. a lactating mother, allowing an older child to suckle is a matter of opinion i feel. personally i wouldnt either. but a non lactating mother allowing a child of six to suck is wrong. completely and utterly wrong, big fat capital letters wrong. even breastfeeding an 8 year old is completly fine compared to this.
Does she have a breast feeding child so that is where the other child has got the idea? Imagine if you had said this and it was another part, of say a man's body...I know it's a different (and more extreme example) but really where I am now (two dc who haven't breastfed for 3yrs) I consider my breasts to be back, finally! to be things that are only touched in a sexual manner I wouldn't dream of letting my children suck them.