Many people ask about what happens with forest fun sessions if it is raining. Well generally I recommend bringing wellies and waterproofs because unless it is thundering and lightning and/or heavy rain we still run the session. Going into the woods is like going into a different land, as long as we are focused on having fun nothing will stop us.
On the days leading up to the events I spend time imagining that the weather will be good and all will be well and it has been. When my sons were young my oldest son was particularly good at ordering good weather (it is known as Cosmic Ordering). He does not use this skill as much now he is an adult, although I continue to do so when I am running outdoor events.
I recommend that you and your child/children imaging that the weather will be good because then there is more chance it will be.