What Documents Are Needed to Convert to Non-O Guardian Visa for Kids After My Husband's Work Permit is Canceled?

Good morning 🔅 My husband has a work permit/Non-B visa until November but he will finish his contract on 30/03, the same day his company will cancel his visa and work permit. I am his Non-O dependent. Our 2 kids have both a student visa just issued. We are planning to convert our visa to Non-O Guardian and apply in Hong Kong. ❓Besides the legalised and certified birth certificate of our kids, evidence from the bank of stable deposit of 500K, and a letter from the school, do we need to ask my husband’s company any specific document other than the visa and work permit cancellation ? ❓❓the kids school told us that because my husband’s visa is valid until November, even if the cancellation is done on 30/03 we still have 21 days by default before leaving the country. Is this correct ? I thought we had 24 hours extendable to a week by going to Cheungwattana. Many thanks and have a good day .