Is it necessary to have two children for the proper development of a child?

I am a stay at home mom.I ahave a 4 yr child I and my husband do not want to have another one. But since 1 yr I am noticing that my daughter has become too pranky, inobedient. She loves to be with childre of her age but at the same time I have noticed that whenever she has chance to meet some children of her age she starts misbehaving, strts asking for their toys etc. I try to spend with her as much time I can but she is not changing. I try to explain the resultd of such behavior but she just doesn't listen. She also has speech problem i.e. she doesn't speak clearly especially the words like must,best, or ch sound etc hence she doesn't likes to recite a rhyme or a story no matter how much interesting I try to make them thru actions or puppets etc.Pls help