But I've cracked it (ahem) and here I can share with you the step by step methods required. You will be grateful - this is foolproof.
(this isn't the answer - but maybe it should be! Image from http://www.amummytoo.co.uk/wp-content/uploads/2012/09/loo-lane.jpg)
1: prepare a snack. Not for you - that would be pretty nasty: for the children.
2: take snack and a drink to the children. It should keep them pretty still and quiet for a few minutes.
3: add an episode of their favourite cartoon and the chance to sit with a blanket over their knees whilst they indulge (no? Just my kids?)
Now you are practically GUARANTEED a peaceful plop - but just to be sure take these steps too:
4: tell the children you just need to go and get an additional few blueberries/biscuits/small dead souls for them to snack on and will only be a minute.
5: do not - I repeat - do NOT tell them you are going to the loo. They WILL follow.
6: shut the door to the room they are in, and every other door you have to pass through.
7: resist locking the bathroom door - you can guarantee that the one time you do will be the time the lock breaks and the kids set something on fire.
8: very quickly do your business. Quicker than that. QUICKER, they're coming!
9: get sweaty palms as you try to poo without breathing so they don't realise where you are.
10: wonder how painful it would be if you stop halfway through and just give up on the idea of a poo
11: shout "DO NOT COME IN HERE" when you hear someone rattle the door. A small voice will say "but Mummy I need the loo loo". Swear silently. "Mummy what is fox ache?"
12: try to convince the child to wait outside so you can finish. Finish with a child holding your leg and asking you if they can watch the poo come out of your bottom. Beg them not to climb onto your knee. Wipe with a child on your knee.
13: put child on the loo whilst you wash your hands. Obviously flush it first. No need to be nasty.
14: wonder what the hell that huge bang from upstairs was.
15: think it can't be anything too bad because there was no smashing sound and there is no crying. Ask child on the loo to hurry anyway. Wonder why they insist on maintaining eye contact whilst they poo. Remember the golden days before children where you could not only poo in peace but also have a flick through the Reader's Digest whilst you were in there.
16: hear the delayed screams of a scared child upstairs who has been doing the breathe out cry for at least seven minutes before the gulp of air and scream.
17: panic and try to find a towel - fail to find a towel - wonder where the hell the towels are as you start running to the screams
18: hear more screams behind you as the other child slips and dunks their bum in the loo
19: grab the screamer and do a lightning check for the three B's - blood, bruises, bumps - wonder abstractly what the huge bang was whilst dashing back to the bathroom to rescue the other child
20: wipe a bottom comfort two screamers and go through the process of washing everyone's hands
21: repeatedly should "DO NOT PUT THINGS IN THE LOO" whilst wrestling one child away from the scalding hot tap and watching the other put things in the loo
22: cry when you see the broken antique table that caused the huge crash - apparently the child had tried to stand on it.
23: suddenly remember the entire loo roll in the loo and go to rescue it, with a child attached to each leg. Wish you were allowed gin in the daytime.
Ok...not so foolproof after all...sorry, I have no answers.