See about making both of them grow. You believe in fairness, and it just wouldn't be fair to grow Vivian without at least giving Ashley a chance to grow with her, or vice-versa. Ashley offers you and her mom tall glasses of lemonade, and you happily accept even as she presses her body much closer into your personal space than she does with her mom. Granted, Vivian has two very large, wonderful obstructions for Ashley to be mindful of... But you almost swear Ashley was trying to flirt with you subtly. You sip your lemonade and wait for both mother and daughter to finish their drinks before giving them the news.
"Ms. Vivian," you start, but are cut off by the very same woman who says, "Please, David, you needn't be so formal... Just call me Vivian." You begin over again, making the correction. "Vivian, Ashley, I...have something to tell you. I -" you're cut off again, this time by Ashley as she says excitedly, "You...really like me? I know you think my mom is hot and all, pretty much every boy in school says so." Vivian chuckles and waves her hand as her face becomes flushed and she responds, "Oh ho ho ho s-stop... Besides, you're interrupting David, dear."
You blush at Ashley's outburst. You never suspected that she actually really liked you, too. You poke in Vivian's mind with your powers, and - WHOA! If that knowledge were to fall into the other guys at school...they'd be after you with razor-sharp pitchforks! But you pretend to have not read those thoughts as you say, "Well...y-yes, that too. You are easily, in my opinion, the second hottest thing to ever walk the Earth. Your mom is THE hottest, but it's very close. But I also wanted to tell you two that I...am basically omnipotent. And I would love to make you two grow...if, uh, that's okay with you..."
You feel your heart beat as the air becomes so tense you could cut it with a knife while you wait for their reaction. To your surprise, Vivian answers honestly, "Well I could always use to be a bit bigger...hehe, or a lot." Ashley adds, "You'd do that for us? Awesome... But, what about the neighborhood and stuff? I'd love to grow bigger, but not if it meant someone getting hurt in the process...Could you -"
This time, you cut her off by finishing what she was about to say. You had already been thinking it, after all. "Ensure that no one can be hurt by you two as you grow? Please. I can do better. Nothing and no one will hurt you nor can be hurt by you two as you grow or even when you stop. It's easier than trying to remember everything down to the finest detail to restore it myself, which even then may be distorted by how I remember it."
You also point out, "Oh yes, I'd like to mention that this is COMPLETELY voluntary. If at any point either or both of you wish to stop growing any further, that's fine by me. If either or both of you wish to return to your original size, just say the word and it'll be done. Believe me, having the scorn and hatred of the two most beautiful things I have ever laid eyes on in my entire life is NOT something on my want list. So, shall we get to the growing or what?"
Both Vivian and Ashley give you the go-ahead and you concentrate your mind on one thought: making the two beautiful ladies grow...and grow.