I ran a bunch of classes at a school a while ago. My students were older, but lessons to be learnt:
Do craft: I did lots of origami. This is beyond 5 year olds. But what about this: attach a pencil to a string and then attach the string to a point on a piece of card. Now observe that by drawing then pencil with the string taut you get a circle. If you fix the string in more places you can get different shapes. You'd need to do a fair bit of prep for this of course.... You could also do something connected to symmetry. Use a mirror and ask what it means for two things to be the same. Distorting mirrors could be used for comparison. Drawing half a butterfly in wet paint and then folding it in two to get the other half. That sort of thing.
Do magic: I did a binary numbers one: I got someone to think of a number and then showed them cards and asked them if the number was on the card or not. Then after 8 of these cards I told them the number. The trick was that each card corresponded to the numbers 1 mod 2, 2 mod 4, 4 mod 8 etc. Even just finding a number on some card may be too hard for five year olds but the principle of presenting your material with some theatre is sound: it can make anything appear to be a little magical.
Come in character: You could be the Numbers Wizard or King Triangle or something. Wear a cape covered in numbers, have a pet rabbit called Cubey, make moo-ing sounds. Whatever feels right to you :-)
And the suggestions above for moving around a lot, avoiding `giving a talk' as such but being interactive are all totally sound.
Good luck!