The Kid grids are hard to find, but here's a way to find them easier:
Cavy Cages Forum - Grids with smaller openings http://cavycages.com/forum/showthread.php?t=1672
When someone orders a cage that has baby piggies, I usually just throw in some extra coroplast to raise the edges all around 7-8 inches like cavylover suggested. That way it's temporary until they are big enough to not get their heads stuck in the regular-sized grids.