I like all the above except I'm not a fan of Seren and I think Iris and Eloise are too similar in sound to Isla. My favourite with Henry and Isla is Emmeline... Second choice Carys and third Imogen.
I listed some other suggestions in another post...My top picks with Henry and Isla would be
Elspeth (Elsie) or
Lillie/Lily would also be gorgeous!
Imogen - One of my favourite names, I like it with your other children's name.
Carys/Cerys - Not keen sorry.
Eloise - This name is lovely too.
Iris - I think this could make a pretty middle name.
Seren - Not keen on this sorry.
Emmeline - Absolutely gorgeous name, love it with your sibset too.
May - I prefer it as a middle name.
Thanks for your comments, all opinions bad or good are gratefully received. :)
Isla Rose...ahhh love!
On your list...
Imogen : this one gets love on here but to me its just musty. I only see it as a name for someone 70+ yrs old.
Carys : like! It grows on you.
Eloise : this is definitely one that grows on you too. I didn't like it at first, but its really making a comeback. If uniqueness matters to you, I don't know that is pick it bc so many "el-" names are popular right now.
Iris : my favorite on this list!!! But I do think its far too close to isla.
Seren : this one is so cool and I love that it means star.
Emmeline : absolutely adorable with a touch of charm!
May : it just falls short for me.