oh dear - number one, its a bit late for bedding plants, not sure the garden centres have any left, no. 2, we're in drought, so under a cherry tree will be incredibly dry normally, never mind right now, and number 3, cherries are shallow rooting and hate root disturbance. Suggest you get yourself some pots and plant busy lizzies or fuchsias or both in them and stand them under the tree and keep well watered. They will cope with the shade okay.

I agree with Bamboo...

Begonias (fibrous rooted and tuberous) grow well in pots and can stand shade too. Maybe have some ferns as a background.