I have 3.5 y.o twins and whilst theyre generally quite well behaved (their first words were "no", so ive been "disciplining" from the very get go, saying no, no smile) ), they can play up, and gee each other up. I either smack on the hand (yes, in public. Ive even done it in front of a cop, and he just smiled = smacking is NOT illegal, and ive had so MANY people thank me for bothering to discipline my kids), or i tell them they will go straight to bed as soon as we get home, or i distract (eg. whip out my phone with game apps on it, and they take turns playing), or i give them a snack. Its also a matter of not tiring them out too much, and recognising that even at their age, they still need an afternoon nap. But i honestly cant remember ever either of them having a total meltdown in public. Ive just been firm from the beginning, and them knowing who's in charge.