Fwiw all of the actors they cast as Baela and Rhaena are bi-racial. The casting guideline specified that they had to have a white parent.
One of the big differences in the casting for HotD vs GoT, is that Thrones held a long and exhaustive search and audition process for the kid actors for Arya, Sansa, Bran, Rickon, etc, and were open to kids that have never acted in anything before.
HotD's casting process occurred during the pandemic, so their casting director cast the kid roles directly from UK/Aussie acting schools. And due to the rules of having kids on set (the amount of hours they can work per day, always having parental chaperones, school) they wanted to cast kids that were already acting professionally or were knee deep in prestigious acting schools with teachers the hiring director trusted.
With the pandemic covid restrictions causing delays in production, time really was money and they had deadlines to hit so they couldn't afford a long drawn out casting search or hiring more green actors.
But this approach also severely limited the pool of available actors they could choose from since the kids had to have a white and black parent, be in a prestigious acting school, have English/British ancestry for the accent, and reasonably look like they could be kids of Matt Smith. I imagine the pool would have been even smaller if they also would have specified 1/4 black ancestry, especially since they had to get it right twice, with two different actors for Baela and Rhaena.
This is who they ended up with for the children (Jace, Luke, Aemond, Helaena, Baela, Rhaena, youngest Laena)
Young Laenor was also one of the more difficult roles to cast, so much that they put out a casting call on facebook and passed it around in industry circles, which ended up getting leaked.