Yorkie, the NRCoC states:
"17. Using a return ticket
A return ticket (including a two-part return ticket) is only valid for the outward
journey shown on that ticket if the ticket is completely unused. You may not use
the outward part of a return ticket after you have used the return part."
So your suggestion of buying a Peterborough-York day return and using the return portion first is not valid, and the guard leaving Peterborough in the evening would be well within his rights to ask to see the return portion. If this wasn't presented, or was already gripped, I assume he could charge either open singles Peterborough-York without railcard discounts or alternatively hand the matter over to the prosecutions department.
I would advise the following:
York-Grantham cheap day return £59.60
Grantham-All zones travelcard £67.25
Total £126.85
This covers 2 adults and 3 children, travelling together at all times with a family railcard. Both tickets can be bought on the day of travel from York booking office.
For this combination to be valid, in both directions you must travel on a train which calls at Grantham.
Alternatively, if you can book in advance and don't mind being tied to specific trains, you can get advance singles from York to Kings Cross for just shy of £25 each way for the 5 of you on the day in question. If you go for this, book as soon as possible, and see the seperate thread for the 20% discount offer.
In this case, you would need seperate travelcards, these would cost £13. On the National Express site, as a seperate journey to your York-London, ask for Firnsbury Park to London All zones travelcard.