Be aware that they are probably aware of other autistic children from school and may not see themselves as being in any way like these other autistic children. If the culture around disability at their school is anything other than amazing they may also have a negative view of autistic people, so going from the angle that everyone is different, even autistic people, and that they are valued and liked is a useful starting point.
My 10 yr old has known he is autistic for over six years, so when we had to tell him it obviously wasn’t an issue as he was so young, but he has always had a very positive view of his autism, even though he realises it does cause him (significant) issues too. He does not really differentiate between people who are autistic, non-autistic or who have other disabilities. In a lovely way he seems to see past it all and just see people…although this also means he makes no allowances for anyone at all, behaviour wise!
I don’t think you tell them they are going for an autism assessment, you tell them they are going for an assessment that will help discover more about how their brain works/what things they find hard and easy/why they find it tricky to do a certain thing and their friends don’t and what could help with that. You don’t know that they are going to come away with a diagnosis of autism.
You could try the cbbc series (and book) A Kind of Spark. Autistic author and the two actresses that play autistic characters are actually autistic, as is the actress that plays their NT sister.
Many resources on autism seem to not be for autistic people themselves, and are not quite disheartening. The NAS has stuff on its website that is less condescending, although it’s not necessarily aimed at 10 year olds.