Grow up is signed just a bit like tall, so be careful not to get the two confused.
To sign grow up, move your flat, dominant hand up with palm facing down, from around your hip area to your neck or chin area. This movement emphasizes how much taller a kid has grown since you last saw them.
Teach your toddler the grow up sign when you notice how big your own child has grown, or when your toddler sees a sibling, cousin, or friend who has clearly grown taller since the last time they met.
This is a useful flash card taught best in conjunction with other signs pertaining to how children develop physically, and emotionally and mentally as well.