To answer your questions:
- Yes, you can use your existing PR Card until it expires.
- Presumably they were opened later, but I don’t know. If you’ve travelled more than your kids since the last time you renewed, it’s possible they’re looking into whether or not you both met the residency obligation.
- It’s reasonable – you have 6 weeks – but you should call CIC and explain why you’re calling (i.e. you’ve received two of the four PR Cards).