The Catholic Church teaches that to receive receive Communion, one must be ”...in a state of grace, have made a good confession since your last mortal sin, believe in transubstantiation, observe the Eucharistic fast, and, finally, not be under an ecclesiastical censure such as excommunication,” according to Catholic Answers.
There are times when one may have doubts about being in the state of grace, or just change one’s mind about wanting to receive communion. In order to avoid the embarrassment of stepping out of line, it is permissable to approach with arms crossed (hands to opposite shoulders), as a respectful signal to the priest (or to whomever may be distributing the Eucharist) that you prefer not to receive.
I have seen this done in the case of family groups, where the children have not received their First Communion, but do not want to wait in the pew until the rest of the family comes back.