loveisntblind is spot on, do NOT have children with this man, best cade scenario, your partner is absolutely fine with overt violence in his family, worst case, he will - like so many men before him - replicate that violence on you and your children when you get pregnant/have children.
As maddening quite rightly said, you can't keep the children away. Your partner can do what he likes, go back on any agreement he makes and - if you split up - leave the kids there for unsupervised babysitting.
The Family Court won't even keep child rapists away from their own children. They certainly won't support you in keeping your kids away from FIL in the event of a split. In fact, if you withheld access from your, then, ex partner, the children would be removed from your care and given full time to your ex (and probably handed over to MIL/FIL each day).
That is the reality of what you are facing, in having children with such a man, from such a family.
I'm thinking of keeping them away from him
As above, you cannot. It's impossible. There is no physical or legal mechanism to keep your kids away from FIL as long as your partner wants to hand them over. And he will. Oh he certainly will.
Don't have children with him.