I'm not sure you can prevent an argument- they (or perhaps he?) is determined to get their (or his) way. You just need to be firm about what you want and let them get on with it. You are not being unreasonable. I too think that he may wait outside the ceremony in an attempt to get in by default, but I'm not sure you can stop him waiting where he wants to.
You need to be very clear with them that he is not invited and that he cannot have a role and cannot sit at the top table. And stick to your guns, or you will be setting a precedent.
I would text back " I'm really looking forward to seeing you at my wedding ceremony. To be clear, your DP is not invited to the ceremony and this is not negotiable. Where he waits during that ceremony is up to him. The childminder can pick up your DC's from school and bring them as previously arranged. If you want her to look after your DC's at the party, then her number is xxx and I suggest you arrange that between the two of you. Regarding your DP having a role in the wedding, that isn't possible. The top table will just be me and DH and our parents- understandably that means your DP cannot be at the top table. Love, Kungfu"