I have taken my 3 year old for a number of flights in a PA28. I would recommend taking :
a headset, or similar ear protection, to protect their hearing. If they have a headset you can communicate with them. (Although I have now taken to not plugging in the mic on her headset as she tends to try and interrupt radio calls and everyonewhat their names are and what they are doing.
Someone else to look after them and a sick bag just in case you can't miss all the bumps!
Be prepared to abort the trip and return if they get upset, you don't what to put them off for life.... do you?