Hi Ivancica, and welcome to the Vienna Thread. I don't want to discourage you, but I think there are very little chances they will let your husband enter Canada again as a visitor. With visitors, they want to make sure they will go back and not overstay their visas. The way they assert that is they check how strong his ties are with his country - does he have a job?, does he own any property? But the most important thing, where is his family? When they find out that his strongest ties are in Canada (you and the baby) they will immediately assume he will not go back to Croatia, and therefore his intentions are to come and stay, not to visit.
If you add to that the fact that he already broke immigration laws before by overstaying for 4 years! his past visa, the chances are close to none.
I know it sucks, but just keep in my mind that you are not alone. There are others suffering, families apart for years, etc.
He shouldn't have left Canada in my opinion. If he was still there, you could have filed an inland application, even if he had illegal status. After the sponsorship approval, he could have got another visitor visa based on the marriage with you and stay with you there for the whole process. But on the other hand, inland applications take longer than outland.
I would suggest, in your situation now, the best thing to do is for you to visit him, or even move there with him. It will count a lot towards your application, the fact that you are together, especially because you guys have a complicated application with his children and all that.
Now to answer your question from another thread:
His children will definitely have to get a medical done (the 20 yr old one only if he is not married), and regarding the other issue: is there any court order that took his youngest child away from him? Why is the child under government care? Where is the mother? If his child was taken from his custody by a court order, he needs to get a copy of that.