Yes, your husband could apply for the kids citizenships if he adopts them, see
http://www.cic.gc.ca/english/immigrate/adoption/apply-who-cit.asp I can not really say which is better or worse, faster or slower, less or more paperwork or costly. You'd have to research each option. Since you have already applied for the kids as accompanying, maybe it's best to stick with that for now. There will be no difference for the kids being PR or being citizens. Your husband can adopt them later and apply for their citizenship right away at that point or you can apply for their citizenship in 3 years when you qualify for yours.
For your husband to sponsor your kids is already quite a bit of proof of a genuine relationship since he will be responsible for them financially for the next 10 years. Adoption would go a step further but if you already sent other proof of your relationship, you really don't need more proof, do you?