You could say "sebah el hare ya Abouna" then " izayuk?" that is interpreted: Good morning, Father. How are you?What?!
He may say "Sebah enoor . . . quaiiss owie, shookran" That is: Good morning . . . very well, thank you. "Wi enti?" That is: and you?
Then you should promptly say "anna ma bufhamsh arabi" which means: I don't understand Arabic.
Why burden the poor people with these onerous and unnecessary Arabic lessons! I have been meeting and greeting Coptic priests for my entire life and not once have I spoken to them in Arabic! I'm sure he knows how to speak english...
Generally, we call our priests "Abouna", which simply translates to "our Father." You may simply address him as "Father" and then "Father X" once you get to know his name.
I'm sure this priest will be delighted to have the cross in his hand kissed. If he has just finished a Liturgy he may even have some qurban (i.e. holy bread) in his pocket for your kids! I'm not sure he'd be automatically inclined to give some (as he doesn't know anything about you, and I certainly doubt he'd be anticipating that your kids would like some in the first place), so if I were you i'd tell him that your Orthodox and warmly explain how your kids often associate kissing a priest's cross with receiving some qurban. I would expect then that if he doesn't have any qurban with him on that day that he'd bring some the next time he visits the park!
For us Copts the protocol for greeting a priest is usually to kiss the cross in his hand, to kiss his hand, and then to quickly place our forehead on his hand as if to take a blessing, but this is certainly not required or even expected of a non-OO, so don't feel obliged as if you'd offend him otherwise. I'm sure even you wanted to just shake his hand as if you were greeting anyone else that he'd be more than happy to receive your hand-shake and will appreciate the simple fact you were cordial enough to approach and greet him in the first place.