Adapter to produce a thin stream of toothpaste for the excellent Y-Brush (now available in the US - https://www.amazon.com/dp/B0BDZMY8ZP - not affiliate link).

Only the very expensive version comes with a toothpaste applicator, which doesnt work well, or you can purchase the kids applicators separately. Or use this...

Nodules at the sides help with the direction of the slit.