Where are you from?
Kyoto is too hot and humid in Early August.If you are from an Asian country which is hotter than Japan,it would not be a problem.But if you are from other country, you will experience hottest and most humid summer in your life.
I think it is worst season to walk around.Especially for a 16-month-old boy.I never recommend walking around with him in the daytime.You can never be too careful of Hyperthermia.
As for temple / shrine and garden,you don't need to worry about.Kyoto receives over 30 million tourists annually.They are not so strict about the quiet in famous sightseeing spots.In other words,you can't expect the quietest atmosphere there too.
Do you know eigamura?
Toei Kyoto Studio Park
Though it is an amusement park and doesn't have real temple / shrine ,I think normal five year child prefers it to old religious buildings.