Endearments, or arguably words in general, only mean what they mean to the people they're shared between. To lots of us (and maybe Solas in game) the literal translation of da'len gets in the way. To the characters in Message Sent it doesn't, so I think all we can do is respect what it means to them.
My husband and I occasionally call each other "baby." We aren't infantilizing each other or trying to reclaim power from the last time the other person said "baby." It's just a stupid word. I use the same word to call my babies "babies" in the literal, "You are my baby (and will always be my baby so deal with it)," kind of way. And I've called my babies "baby" in a endearment kind of way. It's the same word. I use it for vastly different relationships implying vastly different things, but it all works because everyone knows what it means in any given circumstance.
As long as everyone understands and agrees on what the word stands for, we're all good. If one side brings in extra connotations, then it's not good. I call my kids "boogers" and they get mad at me. It means the same thing as any other endearment to me, but they can't get past the other meanings (so I switch to Bouguereau and they accept it).
As long as neither Solas nor Ellana consider it to literally mean child, then everything is fine. Maybe to them it is just a "respected elder" and "respected person younger than me." Maybe it started as the literal meaning used as a joke and that became a meaningless name that is now more name than word.
It gets more complicated with the idea that in-game Solas might have a problem with it so it might come off as out-of-character, but, as Niamaduir pointed out, they aren't the same Solas. Message Sent is very AU.
There's also the problem that some fanfics use the word and do intentionally mean all the things that make it creepy, but I don't think anyone thinks that's the case with Message Sent.
So, for me, if neither of the characters have a problem with it, then I'm not going to continue caring that the author is continuing to have them use it.