Bon appetit, eet smaklijk, buon appetito, ¡Buen provecho! and so on.
Sadly we are missing this in English. 'Tuck in' just doesn't make the grade.
In Liverpool we used to have an expression, "Eat up, you're at your aunty's". This meant "don't be embarrassed. Eat whatever you want and as much as you want." But that's not really suitable.
So I'm proposing "Happy appetite!"
Can you do a little better?