A very nice book in French is "Le Chat au pays des Nombres" by Ivar Ekeland. The book tell a story that happens in the infinite hotel on the numbers planet. This hotel managed by the Hilbert is always full. It is a very good approach of infinity for children around 6.

[1] https://www.babelio.com/livres/Ekeland-Le-Chat-au-pays-des-N...