I'm not sure two 10 year olds are going to appreciate the effort, lol.
I had a brainwave, and went off in search of a little Origami book I'd found whilst excavating the bookshelves. Thanks to my reorganisation, I found the book straight away.
It's now in the "charity shop" box, as the instructions weren't detailed enough for a klutz like me.
Google, as my online friend Sybil might say, is my friend. I searched for "easy origami" (coincidentally the title of the rejected book) and found quite a few allegedly easy pieces. I chose a crown. My first go worked well, but the resulting crown was too big for the box. I quickly re-did the voucher so that I could cut the paper smaller, and my second attempt was just about right. (Just in case you want to have a go, the instructions are here
I packed it all up before I thought of photographing it, so here is a pic of the too-large version.