Same happend to mycase. Before 2 weeks to child fly from ur home town, apply at airport immigration counter no.2,( 500 QAR+ ur RP+ur wife RP+child RP with passport copy and one form is there tio write) they can provide return visa. from that date u have to enter within 2 weeks.

Note; From RP expiry date of ur child within 2 months, you can get this return visa. Once enter do within a month renewal.

2, the departure date of ur child should not more than 1 year.

3, this return visa apply applies for who ever stay more than 6 months out of country also.

Qatar immigration given relaxation for family RP only like this.