Are you ready for some grammar? I hope so! One of the reasons students have difficulty with ‘grow’ and ‘grow up’ is to do with whether they’re transitive or intransitive.
If you’re like me, you’ll want to keep grammar explanations simple, and I’ll often try to avoid the metalanguage of subjects and objects by talking about whether we (verb) or we (verb) something. I found doing that with these two verbs was too much of a challenge sometimes, but hopefully it’s still clear.