there is a theory, that in life everyone passes through stages: dependance, independence, and interdependence in all aspects of life.
Dependence is just reling on others for physical(food, clothing, shelter), mental (intelligence), and/or emotional (identity) needs.
Independence is being able to rely on self for mental, physical, and/or emotional needs.
Interdependence is not only being able to rely on self, but also recognizing that others may be able to pathner with you to provide increase your independence and possibly even society.
All of this to say, a child must go through all stages and surpass them when ready. Sharing within childhood is usually a sign of emotional interdependence, where the child understands that temporaryly giving up a toy will not threaten their immediate survival, will not rob them of other joys/toys, and will increase their joy.
Force does not work to help a child develop into a interdependent person. Try instead to go through the stages with the child. Give the children equal shares of what they treasure at certain times, consistenctly without interference. Small treats, toys, etc. just for being themsleves. As the children become comfortable with the fact that they will not be without, try giving the children fake money for good behavior and allow them to “purchase” a toy with what they have earned or trade-in an old toy for a new one of their selection. This way the child can feel that they can get a toy for themsleves and also rely on you, if necessary.
If these two stages are accomplished (in time), allow the child to exchange their toys/treats (one at a time), of equal value (to get other children involved). Eventually as the children get use to not only getting items for themsleves and learning to get along with others, they are ready for stage three-sharing. This gets tricky, but in the case of toys, try obtaining two or more toys that are all equal in fun or parts, but will be better together (waterguns, multiple player video games, legos, etc.)
The key is to not leave any child out of the activity and to make sure that if alone each child can have fun with their toy, but together they will have more fun. If possible, allow them to figure it out for themsleves.
Sharing is a step by step process, and it starts with self. If a child does not feel left out, cheated, or without, he/she will not have a problem “sharing”. Do not make teh mistake as a parent/teacher/etc. to only buy one item and expect children to “take turns” this violates the above and fights occur. If birthdays are the major issue (one child, many gifts), plenty of group activities (distractions) work wonders, even teh most selfish child will grow board with him/herself and willingly join in!