Not a trick but something that children often find very fascinating. All males have STRAIGHT arms and all females have BENT arms! Roll up your sleeves, arms out straight, palms up, imagine a dotted line down the middle of your arm, you will then notice this anatomical fact.
Another one for children. Ask if any of them can stand on one leg for 10 seconds or more. They usually shout back "easy!" The rule is they have to outstretch their arms directly in front of them, close their eyes, stand on one leg and then count! Most children fail after about 5 seconds, it can be done of course but they often need to practice.
The best family game I've ever seen on a campsite which made most children scream was on a campsite where the family, children and their campsite friends played the game 'Long John Silver' - the man with one eye, one arm and one leg. The kids have to wait at the back of the tent and are warned NOT to look! Dad (usually) will sit on a chair with one arm in his jacket the other arm out, one leg in trousers, the other one out and a patch over one eye PLUS secretly having HALF A TOMATO in the hand that the child knows nothing about. NB, half of the tomato is most important!
When Dad's ready, Mum or another adult will BLINDFOLD a child, but which one has the nerve to meet Long John? Children have a habit of winding themselves in advance up so Mum usually doesn't have to do much to get the tension going. Once blindfolded she will take the child to Long John and introduce them to each other. A bit of acting is required at this point, Mum gives a commentary of what is going on move by move, second by second, so "Hello, Long John" and both will shake hands. Dad does the "Ho, Ho, Ho, pieces of eight" routine. Mum then reminds the child that Long John only has one arm, one eye and the so the next move is to get the child to feel the empty sleeve and the empty trouser leg. She will then ask the child to feel Long John's eye patch and guides the child's fingers to it. The next move is to suggest that the child touches the eye patch again, but this time Dad brings out and holds out the half of tomato and Mum says "and this is Long John's eye" and plunges the child's finger into it!!! The child usually screams, Mum whips off childs blindfold and he or she runs off laughing. Dad keeps half of the tomato hidden so that the next child can participate. When the children behind the tent hear the screams, their tension rise, pity the last child to meet Long John, a quivering wreck even before the blindfold goes on. All show business, but if done well, lots and lots of family fun.