We have 4 kids in ages 8, 8, 4, and 2. As mentioned by previous posters there is quite some overlap, but we have always been very careful with our kids, and have put them in the safest child seats that we could find.
Now, from your first post, I understand you actually have 2 questions:
- Which seat is suitable?
- How to convince your husband?
For a 2 year old (your kid seems to weight 16 kg, ours were much lighter), we always used a 5-point harnass child seat with side protection. Accepts kids up to 36kg, costs a bit more, but also protects much better on impact. We have something like this (not the same):
Some versions have the conventional 3-point belt going through the child seat and fixating it through the back. The seat is therefore always fixated in the car, and you would put your child in the seat and fixate him with the 5-point belt yourself. The majority of children will not be able to open the closure themselves, which is safe.
Our 4-year old just "outgrew" the above depicted seat, and is now in a seat like this (not the same as we have, just an example:
As you can see, the normal 3-point belt goes over the legs under the hooks of the seat raiser, and the upper part of the belt is guided similarly from under the side-head-protector diagonally downwards to under the same hook of the seat-raiser, preventing any damage to the neck.
The main reason why our 4y/o "outgrew" the previous seat is that she is now old enough to understand that she should keep the belt ON while driving, and she is able to disconnect it herself. Therefore, the 5-point belt chair is overkill for her, and she does not want to sit in it anymore.
Hope this helps with question 1.
Now the second question is much more difficult. I feel that the argument "we will not have an accident" is trumped by any traffic statistics, but obviously your husband has a different opinion. There needs to be only one alcoholic driving into your car, and the story is over.
Rather than fight, I would just go out, buy the child-seat you want (aka 5-point harnass), replace the seat-raiser with the safe child-seat, and be done with it.
I do NOT believe it is "paranoid" to have appropriate child seats for your kids. I believe it is money well spent, and I feel that all parents should want to invest in the right safety equipment for their kids.