I noticed that poeple tend to be nicer or kinder and less judgmental when they know before hand.

Go for it! :)
Each of us is different,
None of us is less.
:)

I would we always do. They are going to figure it out at some point but by telling them they can watch for things. If other children are not behaving or if your children need a little extra attention. We have always had good luck with telling

I have unfortunately had the opposite experience with telling. negativity, people stating they don't work with special needs kids, being excluded.

I absolutely would. The way we live is honest and open and we're trying to get Theo into youth groups and mixing with other children outside school and the one that I approached I asked if it was possible for if Theo was needing time away from the noise or needing to stim or what ever the what-have-ya situation maybe ... there's a "go to person" that'd be able to buddy him for company, either an older young person or a youth leader.

I did say that I was hoping that it'd be a Young Person as it's a way of raising awareness and understanding for the whole group to see and share with their peers out with the sessions. They're going to speak with the young people to see if anyone would like to volunteer and if they've any Social Care students doing placements they're keen to buddy them up too! I was really pleased with the feedback and their willingness to awareness raise!