Rent (800-1000sq ft) - 18000 This should be ok as long as long you stay away from the prime districts. It may mean an older style apartment possibly a condo in Kowloon, New Territories and areas like Chai Wan.
Food + Groceries for 4 - 11000 This is quite a lot to spend unless you want organic foods and need to get baby formula etc. If you buy locally from the wet markets you should be able to afford a domestic helper with this budget with plenty of cash to spare. It also depends on how often you want to eat out at restaurants.
Utilities+Internet+Mobile(for 2) - 4500 Utilities such as gas, water and elcetricity should be under 600-700 a month unless you leave the aircon 24/7. Internet 100-300 depending on speed plus Cable tv channels etc. Mobile plans can vary depending your choice of a simple phone to an I-phone.
Transport (for wife and kids, covered by company for me)- 1000 Sounds reasonable if they go out quite often. Using taxis will obviously mean less excursions but should be ok for short distances
School (for 1 child) 6000 Some international schools will charge a lot more than this. There is a Singaporean International School here but I'm not sure how much their fees are
Misc(For unexpected expenses) 1350
Tax 10730 I'll let PDLM or someone else answer this
May need to budget in clothing or entertainment which can come under unexpected expenses. Also holidays??
Hope this helps.
*These estimates can always be debatable by others in HK depending on their lifestyle and spending habits. The more local you get... the more $$$ you can save!