I bought a gallon of this glue about 3 years ago and am just now using it up. I ordered 5 gallons, so will sell it by the cup as a cup will last a long time. This is a vegetable based archival glue and you can barely see it when printed, just like gum arabic. Sand the plate in a circular motion with the fine side of the sanding sponge or fine sandpaper and a block. You are roughing up the shiny plastic so it will have some tooth to hold the goop or modeling paste.
Stickers work well for very young children as there is no cutting or tearing of tape involved.
Try not to layer the tape more than three layers high. You can also use a thin layer of modeling paste and draw into it. This can give you some interesting effects. Allow the modeling paste to dry thoroughly, probably overnight unless it is a hot day and you can dry it outside. If your plate is rough, use a small brush, totally covering the plate as evenly as possible.