I see nothing wrong with it unless you are surrounded by imbeciles who cant tell the difference lol. Like the above poster said, anyone can forget any name, so if people get mixed up a couple of times, no big deal!
I know a girl who's name is Hailey and named her daughter Kailey.. That's ridiculous.... But Hailey and Kayla should be fine. You'll usually confuse your kids no matter what.. My gramma would get my brother, Nathan, and cousin, Matthew, confused growing up. They're names don't have to be similar to get them mixed lol
Doubling up on the long a sound is definitely going to confuse some people. (Also, what if Kayla starts getting called Kaylie by her friends at school, the way Anne gets called Annie and Tom gets called Tommy?)
Maybe you could find an H name that is close to Hailey but doesn't have the same first vowel sound, such as Halle, or something in the same mood as Hailey but with a generally different sound, like Ashley?