Hey Kishore, hope your parents found travel partners but in case they didn't. I found the assistance from airlines very helpful for my mother last year.

There's a person that escorts from  gate to gate and will walkthrough any required process when there's a layover and terminal change involved.

You can opt out the wheelchair if you think it's unnecessary it's easy to just get on a call with BA and request for escort.

I recommend tipping the escort, since they're of such good help and you'll have a peace of mind and not have to worry about your parents going to the wrong gate or not making to it on time.