Hey there.
"Take care" is idiomatic, and you'll find it difficult to look it up online precisely. Do you mean "bye" or do you mean "be careful?"
It sounds from the verb you're using you want "take care of yourself." Is that right?
I think the verb you're using isn't quite right for that. It looks like cuvaj se needs a noun--what are you watching out for (put into genetive, I think). "Watch out for him" would be "cuvaj se njega."
If you want to say: "Be careful" you'll want to go for "Budi oprezan." For a boy. I BELIEVE it's "Budi oprezna" but it could be "Budi oprezana" for a girl. I don't remember if the "a" is swallowed.
Find a Croatian speaker somewhere. Online dictionaries will only get you so far.
One online option, nonetheless.
Note how this online version tells you the answer is "Biti oprezan." I don't think that's right, but I could be wrong. (If so, please let me know.)
Also, formality will play into both that word and into the child's response.
I definitely recommend getting a Croatian reader (as in a live person) if you're going to include much Croatian. (Is there a reason why you need Croatian? Maybe if there's a large group of immigrants from another country who live in your neighborhood, you can switch your Croat to another nationality?)
Also, unless this man comes from Sarajevo, you need to be VERY careful in terms of which dialect you have him speak.
takodjer is Croatian
takodje is Serbian
Plus, I'm not sure takodjer (which means also) is the right choice. It might simply be: I vi.
I'm not sure. I studied Croatian but it's been a long time, and I'm not a born speaker.
Good luck!