I think you've quickly come across common problems, there. To be honest, I'm not sure all of paganism is appropriate for primary school classes in daytime unless handled very sensitively and with experience. Other people might disagree.
Instead of your chosen topic of "the original meaning of halloween before it got americanised" I wonder if you might simply want to do some activities with the children that are appropriate to the time of year. They are very young.
I just googled 'samhain samhuin samhuinn for children' and found loads of web pages that looked pretty good. To be fair, most of them were American, but I wonder if Americans are just a bit further down the road than we are when it comes to comparative religions in school. I'm sure they have a lot of useful stuff to say on the subject.
I know you haven't got long, but in future you might find THIS BOOK
very useful. It is American. I thought it was loads of fun, actually.
Also, I think I heard from somewhere that the Pagan Federation have an info pack for schools, but I think it might be aimed at the teachers. I'm not sure.
Someone else will probably be along soon with more concrete ideas.