I would just keep at it. Keep it short and simple and make sure that you have lots of "action" songs (Sleeping Bunnies, Hokey Pokey, Fast/Slow Song). They may not join in at first but after a few weeks/days you may find that they start singing (or doing the actions to the songs) to themselves. They may not be participating but usually they are listening and watching. Give it time...

At my house we do 2 -3 short circle times per day (5 - 10min). Usually before a transition, before snack, before lunch and before home time.