herethereandeverywhere I think Aunt Stella has some great answers.
Here are my thoughts.
"I'm dreading the whole bloomin weekend of the christening"
Try not to dread it, it is one of life's experiences, there might be some nice food there and even if you do not have any spiritual desires or aspirations at all you might enjoy some small bit of it. Be positive, if you can, I think it will show in your face and make you feel more relaxed. If you can afford it why not get a nice new knock 'em dead outfit to show you are 'taking it all seriously' as in - I seriously needed a new outfit!
"I'll have to fend off assumptions about my DC from the MIL and all her friends"
No, you just need to answer her questions politely in the broken record style you would use for a child who can't quite grasp what you are saying, but all the while smiling, because it is good for you to smile.
"whilst trying to amuse two very bored under 4s in a church with no help from anyone else"
Why can't DH help? Or will he be up the front?
My DH was a Godfather recently at a baby baptism in a quite highest Anglican church and his role was very important - in that he was up the front/ having his photo taken etc.
The following month or so I was a Godmother in a Baptist Church at a baby dedication, I was not required up the front and no photos!
So in the first instance I did understand why DH left our daughter to me.
That might be the case at this Christening so it pays to be prepared. Take some quiet toys they like, some paper and colouring pencils or crayons (not felt pens, too much chance of damage!) plus a drink and something non-noisy to eat! Sorry if this all sounds totally obvious but we are Christians and our DD is 8 and can only just read so does not join in songs etc at church or most of the stuff aimed at adults and is very bored until she can go out for the children's fun bit! So I am used to pacifying her and making it more fun for her.
Finally -
Stella pretty much said it all but I would add ....
"So, what do you think about [DH] being GodP then?"
It's lovely/nice/great/good/fine (whatever level of enthusiasm you can muster) that his sister asked him and he is happy to do it etc."
"Are you getting DC christened"
"No, we have decided together that when DC is older they can make up their own mind about being Christened/baptised/joining a church etc."
"I don't know why you aren't getting DC christened".
"We aren't getting DC Christened but they might choose it for themselves when older, it will be their choice...." And into the loop.....repeat etc!
As far as "you should hedge your bets", you'll have to come up with your own answer to that one!
I hope it is not as awful as you are imagining! I find infant baptism lovely and often cry at them, they are so much about the start of a new life, and yes there is usually the lovely food and a chance to buy a new hat!