I assume you're in Pok Fu Lam too. You can try the Woodlands playgroup in Scenic Villas. There's also Sunshine House in Chi Fu and the HKPPA in Mid-Levels.
D started at BusyHands when he was 2 and was there for a year and half. Its a nice playgroup, and its perfect till about 3. After which I would definitely recommend kindergarten, otherwise the kids get too comfortable in a small group and aren't being challenged and exposed to new activities and people.