Are LEGO Duplo and Lego Junior sets compatible? Can the pieces be mixed together from the different sets and all be played with together?
LEGO Junior elements have the same connections as the standard LEGO System elements and for the most part are the same pieces - it's just the building techniques are usually simpler and there may be more large elements in the set.
As such, they are as compatible as ordinary LEGO which is to say:
DUPLO blocks can be built on top of LEGO blocks and LEGO plates, and LEGO blocks can be built on top of DUPLO blocks, plates don't really have the depth to cover the DUPLO studs.
On top of that, LEGO and DUPLO elements are scaled proportionally, so one 2x2 DUPLO block is the same size as eight 2x2 LEGO blocks arranged in two layers of four.