Hello The teeth of the child look grossly decayed. First of all what is the age of a child? Because the permanent teeth at this area will appear when the child is almost 7 to 8 years of age, till that time we need to do all the measures to preserve these because they act as space maintainers for guiding the proper eruption of permanent teeth. Mostly pulpectomy or child root canal treatment will be needed. In case the infection has reached to the roots or to the permanent teeth then these might have to go for removal followed by a space maintainer that will be given till the time the permanent teeth are not erupted. So it is better that you visit a paediatric dentist and get a proper treatment done

The upper front teeth are badly decayed and require a procedure called pulpectomy. The main reason for this at such a small age might be due to feeding milk in the night and not cleaning the teeth after feeding him.  Few other reasons might be eating lot of sweets and chocolates, inadequate brushing or not brushing the teeth at all.

These teeth need to be seen in an xray, the decay has spread too deep. I doubt sdf will help at this stage, atleast the middle 2 teeth. Apart from SDF, this type of decay needs strict diet modification and meticulous oral hygiene measure. Please visit a pediatric dentist for further management