Congratulations :)
I met a
Nikias very recently. I like the full name but he was always called
Niki and I really can't stand that so I personally wouldn't use it to avoid
Niki at any cost. I also strongly dislike
Nick. But I do like
Nikias and
Nikolai. With
Nicholas and
Nicole (as well as
Nicola in some countries) having been so common it doesn't feel very exciting anymore.
Leonid is nice but sounds a bit like a chemical substance or a surface cleaner.
Leon is so incredibly common here that it doesn't feel exciting to me.
I live in a country where many variants of
Nicholas and
Leon were extremely common (
Niklas,
Leonie,
Leon) so to me they feel a bit like you were trying to find unique variants because
Nicholas and
Leon were too popular. They feel a bit forced. I know that
Nikias is Greek but my guess is that quite a few people will think it is made up.
Leonid could also be perceived as made up by some people, I know it is Russian.
Even though one name is Greek and one Russian they go well together. I like the sibset.
I feel like they go very well with
Barnabas but not at all with
Hjörtur, they really make
Hjörtur stick out.
Overall they are nice names but not completely my style. I do think that your kids have cool and interesting names! As middle names I would probably choose something Icelandic because both of your kids seem to have one Icelandic name (I guess that Húni is Icelandic?).