I think Kayla goes really well with your children's names. I would be tempted to spell it Kayla, as there is no ambiguity about the pronunciation of the name.
Kaela looks interesting, but I think it is a little ambiguous, some people might be inclined to pronounce it as Kay - el -a. Also Kaila looks good too, but again, may be pronounced ky-la.
... i was about to come in and say the exact same thing, Kaela at first glance i thought you had a totally different name and i was somewhat confused until i realised you meant it the same pronounciation as Kayla.
Kaila does look great but i think yeh again some might pronounce it wrong as Ash mentioned too.

Why not spell it Kayla ... i can't see how that would be a problem when they are 2nd/3rd cousins, if anything how would their Mum's not be chuffed and see it as a great compliment

I love the suggestion of Kayla Eve, sounds so pretty !!

I feel the middle name would be better suited with a name ending in 'e'

I can only think of Kayla Ariane & Kayla Trelise for now but i think a shorter middle name would look better, say less than 5 letters.

Kayla May does sound lovely but are you sure about the look of it, that some might think you are looking to match the ' ay ' in both names, not that should really matter especially if you would really love to use May