Find my Phone for husband's LOTR treasure hunt

I'm organising a LOTR treasure hunt for my husband's birthday tomorrow at his work. He'll be on site when I'm setting up and if I bump into him, the gigs up! Our friend suggested using find my phone on his iPhone so that I know where he is.

I logged in on my phone and thought it had worked, but then it notified his phone. I tried again later after it logged me out and the same thing happened. A code came up on his screen, but nowhere on mine to input it.

How do I get this to work without him knowing I can see where he is? It's been such a lot of work to organise and I'd hate for him to find out last minute!