If you use actual liquid, it's going to get everywhere. Even with the idea of making slime, you'll likely need hand towels or something nearby to wipe off their hands after.
A few suggestions:
Red gelatin (jello)?
Plain water, but with a red submersible light or two to "color" the water? I can't find a decent link but I bought some in both red and blue for cheap at Michaels or Joann's a while back...
Red confetti?
Large (like oversized mixing bowl) filled with red beads to simulate crystallized blood drops. Either cheapy jewelry beads like
this, or the
little flat glass ones used in flower arrangements would be kind of cool to dig your hand into and non messy. Would be my fave option anyway. But this one would require a bit of luck finding these in large enough quantities for super cheap, as filling a decent sized container with enough beads/stones to allow for the hiding of a small object could get expensive. I would suggest checking out some dollar stores and thrift stores for crafting items of super cheap jewelry that could be cut up or for cheapy glass vase stones.
And another more abstract idea would be to get lengths of red ribbon/fabric and fill the bowl/container with them and have the object be a specific ribbon with a clue/vampy thing written on one of the ribbons or tied to one, or like a small scroll tucked in among all the fabric/ribbon. Also check thrift stores or dollar stores for cheap ribbon/fabric since it could also be a waste of money to use pricey stuff.
