I find agents at SVO quite helpful albeit not very efficient / courteous. Having said that I think you are cutting it a tad too close. Your parents will most probably be arriving at SVO F and departing from SVO B. MCT for this connection is 1h 25 minutes.
I have never done it myself but from what I hear getting from F to B could be anywhere between 20 and 40 minutes depending on their pace and time of day. With gates closing 20 minutes prior to departure and 15-20 minutes to clear security/walk to the gate, it only leaves 10-20 minutes to clear immigration. Good news is that SU shall (but unfortunately not always will) automatically rebook them if their connection time falls under 80 minutes. If not, they are pretty much at the mercy of gate agents/ground staff. Do your parents speak Russian at all? If not, would it make sense to travel to SVO to meet and fly with them (I would recommend booking cheapest outbound and fully refundable inbound flights). Alternatively you may consider ordering service for passengers with disabilities. I agree with the previous poster, it is doable, but quite challenging in an unfamiliar airport even for a seasoned traveler.
I wouldn't worry about luggage - HKG is whitelisted, so their bags will be checked through to LED.