Thanks everyone.
To line up the buttons I put small black "+" on the artwork at the centre of the buttons, placed art over the wood, then marked the spots with hammer and awl. I'm using forstner bits, which make clean holes, but tend to drift if using a hand drill, so I bought a drill press, ..... The pink buttons are the Sanwas, meant for metal panels, but the plexi holds them in well enough. The black button and t-molding are the only things holding the plexi down....seems to be good enough for now, but we'll see what happens once the kids get a hold of it.
It was harder to get the t-molding slot in just the right place - but thats what scrap pieces of wood are for.
Never thought of using black t-molding....will consider it next time.
Thanks