do we need DATV visa?

[vsa_hlp]

My parents are flying out of India, Indian nationals with Indian passports

Here is their itinerary:

Delhi -> London -> Munich
Munich -> London -> NYC
NYC -> London -> Delhi

They have a visitor visa to US and a Schengen visa.

Do they need a DATV visa?
Is the trip to NYC considered part of the journey while going to Munich the first time?

I would really appreciate if you could point in the right direct to find out.

I have called British airways / embassies / travel agents and have not got any consistent answer.

Thanks in advance

PK