All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we thank and praise Allaah Who guided you to Islam and we ask Him to make you firm and steadfast on His Religion until the Day you meet Him. We advise you to seek beneficial knowledge and perform good deeds.
As regards your question, if your husband has divorced his ex-wife an irrevocable divorce with major separation or that her waiting period from a revocable divorce has expired and she became irrevocably divorced from him with minor separation, then she is a non-Mahram to him, and he should deal with her as he deals with all other non-Mahram women. So, he should not talk to her except in case of necessity, like for asking about the child whom you mentioned in the question. It is the same thing whether he contacts her or that she contacts him as long as there is a need for it. However, his talk to her should be according to need.
In case she is still in her waiting period from a revocable divorce, then she is still his wife, and he is permitted to talk to her for a need or without a need. Nonetheless, we advise your husband to respect your feelings about this, so he should not do what may provoke you. For more benefit, please refer to Fatwa 88317.
On the other hand, it seems that you are very jealous, so you should endeavor to repel this jealousy so that the devil would not make you sad and sorrowful, and harm you by this, or that he [the devil] will endeavor to spoil your marital life.
Hence, you should know that there are some details about the matter and that the general statement of those scholars about this matter is inadequate.
Besides, you should know that a wife is not permitted to ask her husband for divorce except for a sound reason. Indeed, we mentioned in Fatwa 88408 the sound reasons for asking for divorce, so please refer to it.
Therefore, it is not a sound reason for you to ask your husband for divorce due to him contacting that woman unless you are harmed by it. In case you hate your husband and you fear to be neglectful about his rights, then you have the right to ask for divorce in return for compensation that you pay to him and this is what is called Khul’. For more benefit about Khul’, please refer to Fatwa 89039.
Allaah Knows best.