As I made the move from adult to children's poetry, it was hard for me to get used to double spacing poems. It looked so weird to me, since the visual presentation of poems is so integral to the reading experience. A sonnet just doesn't look like a sonnet when it's double spaced. But as I've formatted more children's manuscripts, I'm getting used to the double space approach. I still dislike it, though, in particular because it (more or less) doubles the number of pages of the manuscript.
Someone asked if you can put more than one poem on a page. I don't know the "official" answer, but I have several poems that are just four lines, and even a couple that are just two lines, and I put more than one on a page.
Lil's advice is the most sensible. Follow the guidelines, of course, but the main thing is to come up with a sensible, professional, tidy presentation of your work. I've had my work rejected for any number of stated reasons, but no editor has ever mentioned bad formatting as a reason for not publishing my work.