As ItcheSrulik said you should use kosher food. Whenever I visit my irreligious relatives, we either bring deli or lox and bagels for everybody.
If it will strain relationships with them then you should try to set up the meeting so that the temptation will not arise. You could suggest to meet at a park and picnic or try some vegetation thing. Or you could come just for dessert (if big family get together that many relatives will be present) and stay for a couple of hours.
If all else fails just tell you child that All the food is poisoned.(just kidding in case you couldnt tell)