In Eternals, this doesn't make sense to me because it just seems weird why Sprite is the only 'child' and she's talking about how she's in love with Ikaris.
Sprite is not a "child" as such, she's millions of years old and merely has the appearance of a child.
She's known Ikaris for thousands of years and despite her physical form there is no particular reason should should not have feelings for him.
We are not told why this is the case within the movie although it's possible that this has been done (out of universe) to emphasise her difficulties in fitting in with human society and highlight her "sacrifice" in giving up her immortality to be human.
Just gonna lift this verbatim from the source, since there's no way to really reword it better:
WHY IS SPRITE A CHILD IN ETERNALS?
Played by Lia McHugh, Sprite was designed by the maker of the Eternals – the Celestials – to look approximately 11 or 12 years old.
Even in Marvel Comics, Sprite is the youngest looking Eternal out of the group of heroes – even though Sprite is depicted as a male character in the source material.
All of the Eternals were created at the same time and made immortal, meaning they all carry the same age and never grow older.
There is no definitive reason given in the comics as to why Sprite was made to look younger than the rest, but this decision does impact the character’s emotional drive.