Hi !!!
I think that nature-themed names (and word-names in general) are always unisex.
Hazel has not a feminine sound, it is just traditionally feminime.
In my opinion it could be nice on a boy as well.
Allen and
Kieran are only masculine in my mind. Why not using Allene and
Kiera?
Allen could be a surname, true, but it is a variant of
Alan while
Kieran is a variant of
Ciarán, both clearly for boys.
Personal Name Lists
https://www.behindthename.com/pnl/125456