Welcome to the first installment of "Healthy Children," a regular column on Wakfield Patch where Dr. Tien Lan Chang, chief of pediatrics at Melrose-Wakefield Hospital, will answer your questions about children's health and parenting. To submit a question, e-mail Wakefield Patch Editor Sara Jacobi at sara.jacobi@patch.com. Please include your first name. Your email address and any other identifying information will remain confidential. Here's this week's question: Dr. Chang, For about the past 6-8 months, my 3-year-old grandson has had difficulty beginning sentences, especially ones that…