In my experience, it's always for the best to wait for few days after the exact date of eligibility, to apply for the citizenship. I've seen numerous cases where people have just rushed in, applied on the day or the next when they were eligible and got cancelled after paying the money, because the calculation was either wrong or due to some other issues. I personally waited for 3 weeks before I can apply, but suggest, atleast a week, to be in the safe side.
Your husband can apply for all of you in his Immi Account. You and your husband will have two different applications and either of you can include your 2 yo in your application.