Since you're using tubs why not just drill them with small holes for drainage and use no bedding. Swap tubs daily one clean, one dirty. Baby beardies are going to poop a lot and then there's all those dead cricket parts.... all the beddings you suggested you might use and that have been suggested by others will be hard to keep clean and your bacteria levels could get quite high. No substrate would make for easy cleaning and too much humidity would not be a problem. Read some of the earlier posts on this forum and I think you'll find that the general consensus is that baby BD's be kept on paper towel or newspaper. Again with your tubs I think that any substrate would be unnecessary.
Another thought is to placement of the tubs and temperature control. If these tubs receive too much direct sunlight they could become solar cookers since there wouldn't be much if any air circulation even with a screen lid. You could remedy this by cutting some vents in the ends low down and cover them with screen that you hot glue in place. At least this would allow for better air circulation and temp control.
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Hope this helps.
BigT