The easiest way I have found tell them that god needed a new butterfly for the garden and chose their little brother/sister then choose the brightest flower in the garden (the one most likely to attract a butterfly) then every time there is a butterfly on it tell him it is his little brother/sister coming to visit it will work for a couple of years until he is old enough to understand properly .
For my older children it was easier I just told them that their brothers were born to soon and couldn't survive because they were to small and asked them if they wanted to do something in memory which then gave them something to focus on they chose to build a memorial garden in the back it took them 3 years to complete by which they felt pride not greif .
I hope this helps