If you want them to see Montmartre, but want to limit the amount of walking that they need to do, you could consider doing a private tour of that neighborhood by car. 4 Roues sous 1 Parapluies has an option where you can get a driver and have a custom tour of Paris by a 2CV Citroen car. I believe Viator offers similar 2CV Citroen car tours.
If that's out of the budget, consider taking taxis or buses where you can. Once you are up around Sacre Couer, there are a number of streets that are fairly flat which you could stick to, then get yourself another taxi if you want to just see other parts that will be too tiring for your parents.
The Paris Metro can also be a great deal of walking underground and climbing up and down stairs which can tire out older travelers. Also, if you go to Pere Lachaise, make sure that you start from the back of the cemetery (not the main entrance in the front) so you walk downhill rather than uphill if you are with your parents.
If you are really concerned about the pickpockets targeting your parents because they are old, then take precautions and you carry the bulk of their cash and credit cards in a money belt for them. The pickpockets, naturally, don't like to draw attention to themselves. It's the scammers that are more annoying around the major tourist sites, but especially around Montmartre. They seem to really like hanging around the area between the Paris Metro stations and the base of the funicular, as well as up at Sacre Couer on the stairs. The "friendship bracelet" guys can be particularly aggressive as they are hoping you slow down long enough for them to slap some string on your wrist. Don't try to be polite with these guys if they are trying to get ahold of you or your parents wrist. You'll also see the 3 card monty dealers, petition girls with clipboards, and the ones who pretend they are deaf/mute. None of it is a reason to avoid going to Montmartre or any other major site. Just make sure your parents are aware of it all in advance so they aren't stopping to engage with any of these pests.