Hi Stuart,
Sorry for the delay responding.
The cheapest day travelcard you can buy is for zones 1-6 off-peak and this costs £5.40 more than the adult zone 1-3 daily cap. A single riverboat fare for the Central and East zones costs £1.20 more using Oyster or contactless than you get if you show a travelcard, so it doesn’t look like it’s worthwhile. Prices are going up in two weeks, but the point will likely still stand.
I’m not sure how much a taxi from Cutty Sark to North Grenwich might cost, but given you’ll likely have capped anyway why not use the DLR to Canary Wharf instead.
Has the 13-year old got a zip card?