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.

In general, finding meringue powder is no harder than finding powdered egg whites. Both are available in some supermarkets, in all cake-oriented specialty shops, and online.