Hmm... I actually remember reading something recently (re Airplane travel) that vaccinations were ok to bring, since they were for the health of the child. It might be the same since it is personal use as you only have enough for your child. Besides, the odds of them searching your bags are probably not so great, as you are a family, therefore low risk. But I haven't taken a long distance train from HK to mainland so I can't be sure, sorry.