Finally went to some trial classes (Blossoms Early Mandarin, Kidsland, Better Chinese...) and enrolled in one that suits our needs best. Overall speaking, my child enjoyed those playgroups quite a lot, especially the circle time. However, in terms of exposure to Mandarin, I think we can benefit most from Blossoms as there's quite a lengthy time for teacher-led activities like songs and games. They also provide home-support materials like word sheets with pictures (which I enlarged to make a wallchart to display at home) and accompanying online sound track at their website. My little one has actually picked up some Mandarin words after attending a few sessions!