In heraldry, one person (the head of the family) owns the plain coat of arms, and their descendants mark their versions of the same design with "brisures". There are traditional symbols for the first to the ninth sons, and in Canada for the daughters as well. Here is a chart I made for Wikipedia
For me this is quite straightforward: I use a label of five points. For Rio this is less obvious; if she had ever wanted to use a heraldic symbol of her own, I would have supposed she could mark things with a label of five points and a heart (at least, in the Canadian system), but it's not very satisfactory.
While walking in this morning, I passed the Ministry of Justice building, and on the front they have a statue of Hope. You can tell it's Hope because she's holding an anchor. (The symbolism is that hope holds fast to what is hoped for; without hope, in despair, you are thrown all over the place like a ship in a storm.)
It occurred to me that an anchor isn't one of the standard brisures, and that Riordon's name is Riordon Hope; so although the College of Arms possibly wouldn't think it was all in order, a reasonable unofficial emblem for Rio, should she ever be interested, would be my grandfather's arms defaced with an anchor:
(Possible precedent: Mary of Teck, whose use-name was May, used to use the Royal arms surrounded with may branches.)
When she wakes up, I'll ask her whether she likes the design. What do you think?