at 19 months, she cannot yet distinguish the two languages. that won't happen for another 1-2 years! just leave it and repeat what she has said in english.
I'm going to disagree with Cara's point here... Studies have shown that some kids can distinguish between the two languages from an even earlier age.
I'm no expert but having done the research project, I'm happy to share the strategies that have been shown to work with expat families in Hong Kong not limited to Chinese/English. The research project was on bilingualism in HK amongst expats particularly expats who are trying to maintain a language not spoken in the "resident country" and found the following:
(A) The only way to succeed like a lot of the mums have mentioned is through perseverance and persistence i.e. one parent one language strategy, creating language domains e.g. playgroups in that language, church service...
Even so, there is no guarantee for a child to become an active bilingual - speaks both languages. There is no set system in which a child metabolises language as Cara mentioned before... how one child processes the language may be different and may result in different outcomes... e.g. passive bilingualism - understanding but not using the language. Lots of Australian migrants end up being passive bilinguals.
For mums who have managed to produce active and equal bilinguals! Congratulations! Not an easy feat given that studies have shown that it is common for a child to be better versed in one language than another.
Studies have also shown that where a parent uses a language which he/she is better versed in, the success rate is higher e.g. If German is not your mother tongue, the outcome of raising a proficient German speaker is not likely because you'll screw up the natural grammar of the child. In our family, we use only English or German (between father and son). Our son prefers English when we are both around but switches to German when it's just one-on-one with dad - it has become their secret language.
Cara's strategy has also shown to work - correct me if I'm wrong Cara with regard to your approach - English only (or most of the time) and sending her son to a Cantonese school. I've seen this work with another non-English speaking family. The parents sent their child to an English kinder and the results were simply amazing!
Bilingualism is complex and there is no set answer. Just provide the opportunities and let it all pan out.