This is an age where children have mastered wanting something, being able to physically get it, but not effectively using words to get their needs met. Often, a person who has words will resort to using his body to get his needs met. We can be at a loss for words in the heat of the emotion of wanting something to happen NOW! There are less physical ways to get our needs met, but we need to model them for our children to learn them. And they will over time.
What I would do is to offer more exciting toy alternatives to the younger child. Most often, the younger child is more easily redirected to a toy that *fills* her need. Older children have more specific toy preferences. This models the action that you want your child to adopt! A child will need to see this modeled repeatedly, consistently and effectively for it to be learned as an alternative to grabbing. I would also model the words that I would want someone to use if they wanted a turn with a toy. 'I want a turn, would you trade for this toy?' I wouldn't focus on anyone's actions as "mean". By sharing your understanding of the child's underlying needs, he can relate the feeling of "I want" with the words "I want" instead.
Often behaviors occur due to a child feeling overwhelmed in an environment. Our son is an introvert, we found that by arriving early, or late, or leaving before everyone else, or finding ways to have reconnecting time during the playdate, that ds could *fill* his needs for space and down time. Obviously, this entails actively being attuned and attentive to the child's cues of becoming overwhelmed. There are probably body language cues that are observable before the grabbing starts. Our son would become less able to negotiate and take turns, start getting louder, start moving his elbows out to create a space barrier around him, begin to wander off to quieter areas, etc. I watch for these signals as his emotional barometer and help him to read these cues for himself. I will state my observations and seek clarification about what our son is feeling. 'You seem to be needing some space, do you want to go outside (another room, the car, the porch, the driveway, the kitchen table, etc.) and play with this?' This offers him the template against which to gauge his own emotions before they become overwhelmed. And it models tools of resolving our needs proactively. This is active parenting. And it is work up front but gets easier.
I didn't get much socializing time (and I am extrovert) at crowded playdates or events. Now our son (at 5) is able to be attuned to his own feelings and verbalize them in constructive ways with words. Not perfectly, but I also avoid other things like activities late in the day when he is more tired. And I am conscious of offering food and inquiring about hunger messages that his body is communicating to him. By suggesting possible reasons and actions for our body's sensations we become empowered toward self-awareness and self-control.
I would model patiently asking for the toy, offering a trade, bringing more to share, demonstrating generosity, restraint and self-control.
"Wow! Big child, you really want to play with that toy! Did you want a turn?" Pause waiting for Big child to feel validation of his underlying need. Demonstrating an alternative tool (communication) of getting his need met without making yourself or the child an adversary.
"Ds was playing with the toy." "Ds were you done playing with that toy? You want to play with it some more?"
To Big child: "Do you want a turn also?" Pause waiting for Big child to respond that indeed he would like a turn.
"Ds, I understand, you were playing with the toy and you feel sad now?" Pause to validate that the child's experience and feelings are important and valid. Big child has opportunity to see the *impact* of his actions. "You didn't like having the toy taken from your hands?" Again expresses the action and the impact so that a cause/effect is self-evident.
"How can we work this out? You both want to play with this toy. Do you want to take turns, would someone like this (alternative) toy while the other takes a turn, how do you all think we can work this out so that you both get what you want?" Then LISTEN. Big child is probably stunned that *he* is being treated with respect. Ds will understand that the parent is advocating for him.
No, this won't solve every toy sharing squabble. But, it is consistent with what I want my actions to model: a peaceful means of asking for what I want. My goal is for both children to observe a peaceful way to work together with me as their advocate.
Other alternatives are to consider the HALT theory related to impulse control challenges. Is the Big child needing some decreased stimuli, perhaps redirecting both children to an another activity, or something sensory like swinging or playdoh. I always have some bubbles in the trunk of my car for redirecting overwhelmed child/ren to a fun parallel play activity. If the toy isn't returned, I would redirect my child to another toy, activity or comfort that sometimes this does happen. It usually helps to move away from the play area, if it isn't comfortable for the upset child. No we can not *make* the Big child share. That only works when the child *wants* to share. Helping the Big child to see that sharing isn't threatening decreases his need to control the toys, ime. I would also discuss the situation with the child's mother so that she can be aware that the Big child is perhaps overwhelmed, hungry, tired, overstimulated, etc. I might suggest that our son has more difficulty sharing when he is tired, hungry or overstimulated and needs some space. Perhaps, helping my child to find the toys that he does want to play with, by modeling asking and waiting for a turn, and then going with him to play in an area with less boisterous activity.
Children do what they know. So, my opinion is to *use your words* and ask for the toy to be returned and support your child to get his needs met in a manner that is agreeable to him.
HTH, Pat