Have you thought about how Rayla's parents probably knew their time with her was limited?
Not in a "we're gonna die someday" kind of way, 'cause every parent knows that the most probable thing is they'll die before their child. But in a way, they knew at some point in Rayla's life they would leave, 'cause they had a duty in the Dragon Guard they couldn't deny or change. And as far as they know, it could be as soon as Rayla was born, it could be weeks till they were summoned, months, decades, years, whatever, but eventually they would've left no matter what.
I think when they did Rayla was 8-10 years old, but what would've happened if the egg appeared when Rayla was only a few months old? The entire timeline would've changed.
Picture that scene, they leave when Rayla was a baby, and then return after years to find their baby now grown up into a child or even an early teen girl.
Returning to our actual timeline. I can imagine both of them, Tiadrin and Lain being fearful of conceiving a child knowing that at some point they'll have to leave. I bet Runaan and Ethari played a huge role in them making the decision to be a little bit indulgent and fulfilling their wish to be parents.
With this knowledge that they could be summoned anytime to the Storm Spire obliging them to leave Rayla, I can assure you they make every single moment with her count. Every bday, every meal, every day of school, they try to be the best parents anyone could ever imagine. Maybe they could've spoiled Rayla a little bit, which, being fair, isn't that bad considering how much she has suffered since they left.
Rayla loves them so much, even if when she was 15 she held a grudge against them, now, at 18 she can admit how much she loves them and misses them and regrets telling the boys her parents were dead.