Union J Harmony Challenge

British boyband Union J pop by the Music Mansion with a harmony challenge for the kids: will they manage to be beautifully in tune or an awful racket? The mums face rough tides when pulled behind a speedboat on a giant inflatable - who will manage to stay afloat the longest? With only three Sing-Off's left, the competition is hotting up - can anyone win a second Sing-Off to take the lead?