The difference between pasteurized, powdered egg whites and meringue powder is the addition of sugar and a gum. You can just as easily make a meringue from powdered egg whites.
Presumably the foam on you spooky drink is meant to be sweet, so that your children and guests don’t get a mouthful of bland whipped egg white. So add a bit of sugar after you’ve reconstituted and whipped those powdered egg whites.
For the equivalent of one egg white, mix 2 tablespoons of Just Whites with 2 tablespoons of warm water, and stir with a whisk for a couple of minutes, until completely dissolved. Then use a mixer to whip the whites to whatever stage is appropriate for your drink – foamy, soft peak, or stiff peak – adding that bit of sugar just before you stop mixing.