Trust me: Your child will not end up at a shrink because you breastfed her while she was a toddler (she may have other reasons, but it won't be this!). In fact, the World Health Organization encourages moms to keep nursing through toddlerhood. Health benefits aside, it's a great way for moms to bond with their kids -- which you've clearly recognized from nursing your daughter. It should stop when you're tired of breastfeeding or your kid decides she just doesn't want to do it anymore.
Which is the nice way to tell the playgroup moms to mind their own beeswax. Whether you nurse at all and for how long are extremely personal decisions that are open for discussion only with your pediatrician. Of course, the ladies are welcome to their opinions, but they should leave the judging to Judy.
Still, if you want to keep the rolled eyes -- and your discomfort -- to a minimum, nurse her at home before going to the playgroup, and bring along a snack for when she gets hungry.
The day will come when she no longer wants to breastfeed (or you just can't anymore). In the meantime, trust that you'll be able to comfort her in other ways when she's older, and take solace in the fact that you nursed your child in a way that you saw fit.