This is why it's all bollocks. My ex used to buy halal if he was going to the butcher, but he was happy to buy and eat anywhere, just not pork. Oh yeah and he drank (like all his friends) smoked (including hash) and cheated when ever he could.
He was (and is) controlling, manipulative, abusive. He made me suffer for years, and then mercifully left when I wouldn't do that he told me any more (God bless mumsnet 
)
For a while after he left I carried on as before, no pork.
Then I realised that I didn't need to live by his rules any more so started eating bacon etc again.
I resisted giving it to ds (5) at the time, but he wanted to eat it, we had a talk, I explained that it was not permitted in Islam. (Again, zero effort from his deadbeat dad to foster the religon)
Ds and I eventually took the decision when he was around 6 for him to choose for himself.
So he eats whatever he likes.
Of course when his dad came back for the first time this summer, he wasn't happy about the pork, blamed me, but I told him it wasn't a rash decision taken lightly... Blah blah he didn't listen. I don't care.
My son can make his own mind up to get religon or not. I don't think he will, he doesn't believe now, and I can't see that changing.
My suggestion to you love is to agree with your exh that he cares for them wrt food in the way he sees fit, and you will do the same.
When they are older, they can chose religion if any, and you'll listen to them.
Your ex was not observant in many many ways, so he's in no position to make comment (he will tho, I'll bet)
If you can't agree this upfront with him, do it anyway.
You are very thoughtful and considerate.