I would love to know exactly what Bilbo meant here – a children’s toy, I hope?

• 07.064 Come on my jack-in-the-boxes!’

Update 2017.09.14: Well, well, well.  This Was Beorn to Nori and Ori who came with alacrity from hiding, so I assumed, of course, he meant “oh, you who have popped up vigorously”.  Well.  That’s not all it can mean.  OED gives:

†1. A name for a sharper or cheat; spec. ‘a thief who deceived tradesmen by substituting empty boxes for others full of money’ (Nares). Obs.

†2. Applied contemptuously to the consecrated host, with an allusion to its reservation in the pyx.

3. a. The name of some gambling games.

3. b. ‘A game in which some article, of more or less value, is placed on the top of a stick standing in a hole, and thrown at with sticks. If the article be hit so as to fall clear of the hole, the thrower takes it.’ (Farmer Slang.)

†4. A street pedlar stationed in a portable stall or box. Obs.

5. A kind of firework.

6. A toy consisting of a box containing a figure with a spring, which leaps up when the lid is raised. Also fig. 

7. Applied to various mechanical contrivances.

So, yes to the game, and used figuratively of people who leap up… but remember that Gandalf’s best-known craft was firework and these dwarves popping up out of the hedge were a little bit of a scam…  It’s not quite a gem word because I’m not weeping… but dang, that man could write!

“Jack-in-the-box | Jack-in-a-box, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/100519. Accessed 14 September 2017.