It is a sign of respect for the pledge itself and for the item you are pledging to.
I assume you are referring to the Pledge of Allegiance in public schools. The answer is no. It is federal law, under the First Amendment's free speech clause, that children may not be required to recite the Pledge. Children who don't want to do it, for whatever reason, can stand or sit respectfully while the others recite the Pledge. Schools are prohibited from forcing children to recite the pledge, and schools are prohibited from punishing those students for refusing to recite the pledge. However, any students who harass those who don't say the Pledge are subject to discipline or even expulsion.
There is no law requiring you to do so. If you feel patriotic and wish to participate in the pledge, then stand, put your hand over your heart, and recite it. You can also stand respectfully and remain silent if you have any objection to the pledge (e.g. some people do for religious reasons, or for political reasons). Sitting while others stand and recite the pledge is your right, but people will undoubted be offended by this so be prepared to engage in a conversation with people about your decision.
There is no Australian pledge of Allegiance. Children do not recite any such pledge in school.
1955
Recite the Pledge of Allegience.
October 1892
yes
School children first recited the pledge of allegiance in 1892.
The children were trained to recite the pledge of allegiance in unison
Uniform military are not required to say the pledge of allegiance. They are required to stand at attention and (if outdoors) salute. Uniformed military may choose to recite the pledge if they are in a mostly civilian group and not participating in any formation or ceremony, but even still it is not required.
active citizenship
The children were trained to recite the pledge of allegiance in unison.