There are 2 in Plymouth plus Savers and maybe a 3rd.
Former Favorites is in North Plymouth off of 3A but she will probably be moving at some point. It is chaotic and a little messy but Tracy will find stuff for you and often has some good finds (got some great Kamuk boots there). She has her phone number listed on the door so I could probably just post it... if anyone wants it.
Updating 2/10 Looks like Just Like New is closing... check their website.
Just Like New is at the shopping plaza at the corner of Carver Rd at Rt. 80. It is very well organized. She usually has very few clothes out of season and also only takes clothing from the current/next season (like now she probably would only take spring stuff). http://www.justlikenewforkids.com/
I've found both to be good on occasion but sometimes they can be overpriced, especially since some of the retailers have some great sales.
Savers is by Shaw's and has kids stuff, adult stuff, kitchenware and household stuff. The profits go to Big Brother Big Sister. They take donations only. It is off of Pilgrim Hill Road (back on the hill past the Shaws). http://www.savers.com/Find-A-Store.aspx
There is a 3rd consignment store in Plymouth along the main street in center Plymouth. It used to be called Finders Keepers and had great stuff (imo) for children and women. This store was really well organized and looked nice inside (small little shop). That owner is no longer there and another woman has reopened a nice consignment store there. She said she will start to have childrens stuff later in the spring, but does not have any right now.