It's important that your child have time alone with you and with friends, because he's reaping different benefits from each play situation.
Playing with other kids is a crucial part of your child's development because it helps build language skills, teaches cooperation, and jump-starts the critical thinking that occurs when children challenge each other's assumptions ("I bet the next block you put on your tower will make it fall down!").
Interacting with you, however, is invaluable: This time spent together offers you both priceless opportunities to learn and strengthen your bond.
With you, your child will be at his best because he's playing with someone he feels deeply connected to and someone he knows cares about him. As you tune in to him, he may be more experimental in his play and express himself with increasing comfort and fluency. Also, the attention you show your child during your special playtimes is key to building self-esteem. For example, when you pretend along with him, you're demonstrating that you accept his make-believe world, that you understand his style, and that something he's interested in is fun and important to you, too.
During play with your child you're also able to offer cues about how to behave in the real world. For example, by waiting for your child's tea to be poured before you drink yours, you're teaching manners. If you talk to his imaginary friends and then wait for their response before continuing with the conversation, you're introducing the concept of taking turns. During the tea party conversation, the language you use will be rich in new vocabulary he can start to try out for himself in conversations with you and his playmates.
Keep in mind that "playing" with your child can take many forms: reading together, going on an excursion, sharing a special meal. This valuable time spent together teaches you about each other and forms wonderful memories you can both revisit for a lifetime.