You asked about song length - don't underestimate kids of that age. They'e quite capable of learning several verses, especially if the song tells a story, or has lots of repetition, or rude bits, or a chorus,or an obvious sequence like numbers. Most of the songs you're being directed to will have one or more of these features. In fact, if you stick to two verses, the song's over almost before they've got the hang of it - it's got to have a respectable length to be learnable.

In any case, they don't need to know all the words, they just need a confident leader who does, and then they'll join in with the bits they know or like.

I still know huge chunks of songs I learned at a very young age, and I don't remember ever making an effort to learn them.