...yeah, forget the sand covering for now (for future ref': playbox sand is fine, just rinse it well),...EoF has good points too,... I hate it because it gets sloshed around when you water, and makes moisture assessment a pain,... if you have the dunks in there, that should do,...recall, it's a biological weapon, a Bacillus spp. bacteria, that wrecks their guts when ingested,... it's a preventative measure at this stage, not a full on extermination run!....
...blending the soils is fine, but before anything, make damn sure you have a read on both soils' pH's (I saw at your home thread A4's advise),... In any case, I'd add some dolomite for longer-tern control and nute' input,.... finer grain size, the better! Several companies make so-called faster acting limes as well, readily available,...