tl;dr: OP, they warn about leaving valuables, yes. They're covering their insurers by doing that. They're not suggesting that the hotel itself is a hotbed of pirates (aaaar!!!) or that it may be riddled by predatory paedophiles. Relax, use the facilities you've paid for (they wouldn't put them in place if they weren't useful or increasing business). Enjoy the wedding. Chances are good that you'll want to go to sleep before your child, if you let him/her join in the dancing!
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Longer version, at risk of being a sermon:
"I wouldn't want random maintenance men/maids/room service people having access to my sleeping child." - from edwinbear.
That statement terrifies me. Do you really think that the world is populated by lurking child snatchers? Is that the world in which you want to bring up your children? Is that the world in which I should bring up my children? This isn't Chitty-chitty-bang-bang Bulgaria...
I would, and have, and will again, leave my children in a hotel room whilst I enjoy "adult time". I've done this since my youngest was about 11 months old. I have used listening services, where available. Where listening services weren't available, I've exercised my judgement and checked on them every half hour or so.
Teaching your children that every unknown adult is a live threat to their safety is a very, very dangerous thing - for your child, and for society as a whole.
It's also detrimental to you - how can one go through life, healthily, whilst believing that every stranger you might meet is a potential threat to you and your children?
I'd argue that leaving your child in a hotel room whilst you do "adult time" is actually doing them a favour. Sleeping in unfamiliar surroundings at a very young age will give them massive benefits when it comes to, say, their first Beavers/Cubs sleep-over. The home-sick kids will be sobbing at Owl at 10pm, whilst yours will be sound asleep, not caring a jot that he doesn't have his mummy/daddy/favourite-furry-friend to hand, and getting up in the morning to kick ass at murder ball (or whatever boisterous game they haven't yet banned).
Teaching them that anywhere which isn't "home" requires mummy/daddy/sitter to stand guard over them against the hoards of potential paedophiles, some employed by the owners of the building which you took them to sleep in - how does that help the child develop into the well developed adult which, ultimately, it is our job to help them to be?