We used to take our two dogs to daycare on weekdays in the US and they loved it, but I wouldn't even consider it in HK. The only places with decent facilities are just too far away, moving dogs around is just too difficult and we have hired a helper to do essentially the same thing at our home. I would also have concerns about how well the facility was run (DB Dog Dayz exculded because of the number of excellent reviews my friends and colleagues have given the place over years).
A couple dog "daycare" places have come and go on HK island since I've been here. There is currently a Three Dog Bakery in Happy Valley that has dog daycare, in addition to boarding and a dog food/ supply shop. The daycare bit there is expensive and not all that in demand, small area and again, most people have helpers to care for and walk their dogs while they are working. There are quite a few informal dog play groups on HK island that I'm aware of in various areas. Some with owners and others with helpers and this seems to be how many owners who care about socialization go about it.
There are dog daycare and boarding centers scattered throughout the new territories - Pet Oasis and the like. Not sure how popular they are, but these facilities are pretty large (big fenced fields, swimming pools, dog agility courses, etc.) and also have boarding facilities. Treking out there to drop of and pick up a dog from even Kowloon on a daily basis would be awful. We've gone from HK island to a place in the NT with a dog swimming pool a few times (maybe 3-4 a year) with our dogs, friends and their dogs, but it isn't a weekly or even monthly thing. Too much hastle to get out to the places with big and nice facilities. Would love a private (or public) dog park type place like this (nice, safe and fenced) on HK island for evening and weekend outings, but the cost of land would be absolutely prohibitive.
In additon to competition from domestic helpers, another thing to consider is how will the dogs get to/ from the daycare? In DB this may not be as big of an issue because it is a relatively small neighborhood, but it may be in other areas. Not everyone has a car, dogs cannot take public transport, and if the owner can afford a car, in many cases they would also have live in domestic help.
Don't mean to be discouraging, but a dog daycare alone would be a tough business in HK. However, a full boarding facility, with a daycare option may be something to consider. Several boarding facilities also offer other services like training and group obedience classes, and assistance with pet import and export, grooming, etc.
Re: pet cafes - places focused on folks with small dogs come and go on HK island in Causeway Bay and the Mid-levels and a few (One Dog Life I believe) have been around for a while. They don't appeal to me though. A nice outdoor pet cafe might work in Sai Kung, but probably not on HK island - the few places with restaurants outside seating already tend to permit dogs (most Stanley restaurants, Peak Lookout Cafe, etc.).