It should be specified that, like so many of the time, their royal wedding was necessary to seal a deal: Richard's truce with King Charles VI "The Mad" of France. So it was indeed a political agreement, not a pervy romance. If anything, her inability to produce an heir in a timely manner might have been a more serious problem than informed consent.

Also, this is apparently a painting of their wedding, provenance unknown.

Why would that be scandalous? Are you assuming he tried to consummate the marriage that year?

Asking as a question because I'm not 100% sure that it's the answer, but:

Wouldn't she have stayed with her previous guardians until she came of age? Was that not the common thing to do in these situations?

OK, a small rant at first- I'm opening AskHistorian and what I see? Anecdotal evidences and Romeo and Juliet cited as a source? Has anybody given it a second thought?

OK, serious answer, but let's start from the bottom. Did people marry so early in the past? Yes. And No. The false impression that people pushed kids into the bed of elders comes from the fact that most written sources deals with the aristocracy who didn't have to think about common folk's problems (described below) but has the most to win and lose in dynastic games- so yes, they married early, but not as early as one might think.

The line between betrothal and marriage was often very blurred, negotiations about dowry could take years, changing alliances could make a deal not a deal and force to change it, personal reasons like parents claiming that bride is too young or having problems with letting the child go also played a part- so in fact "young" was closer to 17-18 than 12.

Poorer parts of the societies married even later- woman had to gather dowry, men had to inherit land or learn trade before he could support a family (for example apprentices were usually forbidden from marrying). When we look to parish records, the age of women is often above 20. And don't forget that you had to pay for church marriage and got permission from your lord so it wasn't in uncommon for couples to live as "common-law marriages" till they could afford proper wedding.

So what about those child marriages? Technically they were possible (for girls above 12) and sometimes they happened when the situation pushed- especially when survival of the dynasty was imperilled. Now my personal observation- due to unusual circumstances surrounding those weddings they made it to the literature and public conciousness. So we remember 12 years old princesses marrying old men, but forget about those boring 18 years old marrying young men. Case from my country- everyone knows that 40 something Władysław II Jagiełło married a 12 years old Jadwiga (their child was born 13 years old after ceremony so who knows what happened in private chambers) but his second and fourth brides were 22 and 17 respectively. And his third bride? Mother of 5, widow after 3 husbands, well over 35. So that's all about anecdotes.

Now let's go to the case- I haven't found details about ceremony, but I doubt that he would actually made out with a child- when the bride was younger than 12 the bedding ceremony turned into purely innocent sleeping side by side. If someone was really eager, he could bare his and her legs up to the knees. After such "consummation" marriage still theoretically could be cancelled, but Church jurisdiction simply wouldn't take seriously a full ceremony with so young girl. And international reaction? Apart from those who actually lost something with this match it was probably "meh", since I haven't seen any trace about moral sithstorm you could expect.

Edit: from the initial shock I forget about resources- Jacques le Goff and his "L'umo medievale" about the age of marriage and "Kronika Kobiet" by Maria Żmigrodzka-Wolska for details about bedding.

Don't worry, it wasn't consummated for quite awhile. The only reason the wedding occurred at her age was for political aims. :)