If not, a pitch pipe is a great little portable tool. You can use it to find the note you want to sing or hum as a starter. If you are shy, you can just play the note for the singers. Often when male Scouters sing with young children, there's a tendency to choose a note too low for them. For the most part, you are basses or tenors while young Beavers or Cubs are sopranos or altos who find it very difficult to sing in your range.
Nobody enjoys doing things they find difficult.
One way to get around this is to ask a female Scouter or one of the youngsters to start the song. Here again, a pitch pipe comes in handy. Every time they sing a new song, use the pipe to determine the group's comfortable starting note for that song. Write it beside the song in your song book for future reference. Then, whenever they are going to sing it, you can simply blow the note on your pitch pipe to cue them.