Dear parent,

I can understand your concerns. This is a very common query in India. The harmful effects of passive or second-hand smoking is very well documented in the literature.
Breathing in second-hand smoke is particularly harmful for children. Babies less than 1 year of age, who breathe in secondhand smoke, have an increased risk of cot death or sudden infant death syndrome. Smoking can also trigger asthma attacks in children who already have the condition. Younger children, as in your child's case, are also much more likely to be admitted to hospital for a serious respiratory infections or frequent cough and colds when exposed to second hand smoke.
Twenty five metres sounds far enough but there are no guidelines as to what distance would be completely safe. Studies have shown that second hand smoke exists in the environment even when you cannot smell anything. If you can smell it, it is definitely there. The maximum harm is when you have smoke within a closed space like a room with poor ventilation. So, it is good that there are no smokers in your home.
For the sake of the neighbour's as well as your health, you could try motivating him to quit smoking. He is killing himself and causing harm to others. But it is easier said than done.
All the best.
Regards
Dr. Saptharishi L G