in water barrier

Hi All,
My wife and i decided early on that we wanted to be able to separate our tanning shelf from the rest of the pool to make a kiddie play area. The plan *was* to use baby barrier along the outer edge of the ledge, but the BB installer said he wouldn't do it - he's afraid it would leak.

Sooooo, if that's not going to work out, does anyone else have any brilliant ideas that we could use to keep Noah on the ledge and not falling off into the shallow end? (btw, the ledge is under 12" of water)

thanks