I think it's a really tricky situation, I can understand both the reasons to sign and also not to sign.
You see it as a slight on the way they think of you, this might not necessarily be the case. I honestly think anyone who doesn't protect themselves financially when entering a marriage is at best naive. They have obviously worked hard for their money and quite rightly should be protecting it, but possibly the way they are doing it is wrong. I don't think any spouse should be entitled to inheritance, or anyone with a decent moral compass should try to access their spouses inheritance on separation.
I would have no objection to signing however I would ensure provisions are in place in case of a divorce to take into account your DC and also your loss of earnings as a SAHP.
No one enters into a marriage expecting it to end in divorce, or when you are happy in a relationship, you also don't consider how badly marriages can end. But all you need to do is read some of the threads on here, to see just how Nasty divorces can be, no one could foresee that in the beginning. You PIL want to protect their sons finances in case possibly you cheat and leave him for a younger man, etc... and then try to take him for everything he's got. It does happen, a lot.
Wills can be as simple or as complex as needed, as you are all adults and have had this sprung on you I don't think it would be wrong to sit down with PIL to discuss their full intentions, and put forward your points for discussion.
For example if he cheats and leaves, you get X amount payout for you and DC. If relationship naturally breaks down you have X amount for you and DC.