The milk teeth will normally fall out by themselves by the age of 7 - 8 years. So there is not much of a concern if the permanent tooth is erupting behind milk teeth. Do not worry. If they do not fall on their own by the age of 9 and the permanent teeth are do not fall into place, you can visit a pedodontist (a dentist for children) or a general dentist for the extraction of the milk teeth.
Next Steps
Stop Worrying. Visit a Pedodontist for opinion.
Health Tips
Do not go for extraction of child's teeth if it is not paining.