You will have higher housing costs here in HK compared with an HDB flat in SG.
And even if your wife will find a job here, you have to consider that she will first have to give up her job and that there is only one income for a while. If you now rely on your family to support you with your kid, it is an important factor to consider (you mentioned you need possibly a maid)
Your company will have to contribute much less towards MPF compared to CPF.

20% more for both the promotion and relocation seems a bit poor considering your situation.