Town Square Research to Practice Statements offer information from theory and research with examples and suggestions for what it means in your work with children. This series of position statements includes topics such as fostering self-regulation, the power of open-ended materials, and the benefits of incorporating the arts.
Talking to parents about your concerns can be difficult. However, open communication is always best. You may suspect a delay, but after talking to parents you might find there are other underlying reason for a delay or you might find parents have the same concerns. This handout from the CDC can be a helpful starting point. Though it was designed more for classroom teachers it has some good advice.