will june be better for traveling with my 2yr kid? at what time crowd will be less ?

Where are you going for travel? Generally, June will be hot in most cities of China, with the highest temperature of 30C or higher. If your kids are not afraid of hotness, it is ok to travel in June, when it is not that crowded.

Just avoid the early May and early Oct. Other seasons can be ok, considering the tourists' number.