24-36 Months

My 2yo DS does not interact with other children and adults as well. It's just like he is way to shy to even say hello in reply to someone's greeting him. Every time we go to a toddler group or a play group he will sit and play either with me or by himself but close to me, he never plays with other children. When we are at home he is a very lively and bright child but when there are people he does not not very well he just closes up. I am worried he will struggle with school too much. 

DD is 2.5 years old and we are thinking about starting potty training. Anyone knows some good books to help?