It’s worth saying that as you are still your ex’s legal next of kin, you want to make sure that there are no other payments due to you - final salary, any other death in service benefits and so on.
They will know it is a difficult phone call - hopefully it’s something that will never happen to you again so they will know that you don’t know what to say, they probably don’t deal with it very often either and they will want to be nice and helpful.
You can ask if they have any additional advice or info for you. Also find out when is a good time to call back if you have other questions.
Ask how much comes directly to you and the dc (important to keep mentioning them!), how much goes into the estate.
Depending on the type of employer might be worth asking if ex had legal cover that could be used to get advice regarding his estate if appropriate.
You’ve done really well in contacting them to get a reply, it’s a good step forward.
Re the life assurance I would start by asking what cover he had through the company, the general way the scheme works, who had he nominated the money be left to.
If it was to his siblings/family/ie not you and the dc - it’s reasonable to ask when he made the decision - if he worked there before meeting you and having dc and didn’t update, very reasonable to ask trustees to take into account that he has young dependent dc that he would have supported if he was still alive and that could do with the money for their education and everyday needs.
If they released the money without taking them into account definitely worth questioning.
If he changed his info and the shareout after you split up - more difficult to argue with but certainly worth a try because they are supposed to do right by any dependents
Good luck.
(I wrote this last night and just realised it hasn’t posted so hopefully this hasn’t cross posted and isn’t too rambling!).