Sounds like a harrowing experience for your granddaughter and for that I'm sorry. I think you should focus on the positive experiences you all had on the cycle paths and probably take her back there in the future. Gymnastics is a great alternative if the altercation puts her off cycling for life.
Regarding the incident; the highway code clearly states that you MUST NEVER ride in an inconsiderate manner.
Ask yourself the key questions:
Was the road busy? You answered yes.
Was I riding two abreast, in a fixed formation? Yes.
Are there frequent pinch points? Yes
Was it a nice sunny day with less than 10 shopping weeks till Christmas? Yes.
Was your speed slower than the average cyclist? Given that she's only 6; probable.
Only you can decide if your cycling was inconsiderate to the other road users.
I'd a great deal of respect for anyone who challenges irresponsible and illegal cycling, but the person who confronted you was clearly an angry knob, who doesn't know the rules about pedalling on the pavements.
Anytime I pull up adults cycling with children I always watch my language and I carry around a tub of strawberry candy lollies to ensure all parties leave on harmonious terms.