Ur pedodontist is right.. the decay is due to improper oral hygiene. Upper front milk teeth are usually lost by age 6 when the corresponding adult tooth starts erupting. The back teeth both upper and lower stay for longer upto abt 11 yrs of age. Hence i would suggest that if it has started spreading , consult your dentist again and seek treatment for the same. There are a lot of challenges for dental treatment at such a young age, but that should.not be a deterrent to treatment. Pls consider treating and preserving the milk teeth as far as possible against extractIon as they keep space for the adult teeth and untimely removal of milk teeth can cause malalignment of the erupting permanent teeth. And if the child is habituated to snacking between meals, consider changing to healthy snacks like fruits instead of chips or biscuits as well as a gap of atleast 4 hrs between every meal.