I originally tried to find a mandarin speaking mainland helper as well, but I was told by agencies that it would be illegal for me to do so. . .and supposedly immigration would never approve it.
However, I now have a lady who does part-time work and is Indonesian/Chinese (HK Citizen), & I end up communicating with her in Mandarin vs. English. I also know one of my son's school friend hired a local helper who speaks Mandarin & Cantonese. . . .
I have also seen Indonesian & Filipino helpers who have previously worked in either Singapore or Taiwan that have picked up mandarin quite well, don't know if that would be good enough for your purpose.