Please have a note added to their/your reservation. Call the airline and have it added to the record. It will make life easier for everyone especially them. If they want to refuse the cart/wheelchair, they can, but if it is not in the record it can be difficult to get help later.
Any travel, especially international is difficult, for young, old, able-bodied, ect... The distances in LHR are vast and after they spend the day traveling they will be tired when they get to Berlin. I am sure they will appreciate the extra thought you put in to ensure they have an enjoyable trip.
Talk to them about it beforehand, let them know the decision is up to them, but you wanted to make sure they had a nice, non-stressful trip. I have found that the "helpers" for both carts and wheelchairs are very nice and accommodating in both the UK and DE.
Enjoy your trip, take your time and don't sweat the small stuff!