Ahh. I know it can sometimes be difficult with toddlers, but they are my favourite age group.

If I am remembering child development right, sharing and taking turns is a concept learned about 3 yrs of age.

When my infants/toddlers take toys away from others, I am a broken record with 'no thank you'. The toy is given back to the other child by the toddler sometimes with help and the child is encouraged to find another toy to play with, sometimes with help. When I help is dependend on the child's maturaty.

With sharing not being a learned concept until closer to preschool aged it is difficult for toddlers to share, some share amazingly well, others not so well. I find once I know the child I know how best to manage their behaviour, meaning whether encouraging them to share is the best option, or maybe setting a timer so that in 2 minutes they need to give their friend a turn with the toy, or sometimes depending on the child I just have to ask them to give the next friend the toy when they are all done. Sometimes situations dictate that a child shouldn't be required to share as sometimes a child just wants to play alone.

I have a little one in care (2 1/2) who will do something similar to what you say, she grabs the toy, says 'no' and tries to turn her back to her friend that is trying to play with the toy. Her behaviour is best managed not by encouraging her to share but my empowering her not to have to share, if that makes sense. If I see her do this, I just gently remind her that the toys are for everyone to play with and when she is all done she needs to pass the toy to child B. This allows child A to know that the toy won't be taken away from her and child B then knows s/he will get the toy in a little bit. As grown ups we know this is still sharing, but I don't quite think she views it as sharing, lol.