Hi 13lucky, yes my son had this when he was a toddler. It really isn't as bad as it sounds, honestly (although I agree it sounds horrible).
My son already had an NG tube in when he had his done. The tube for the pH probe is much thinner, and they surprised me by putting it in the same nostril
.
It is apparently a bit of a funny/uncomfortable feeling when they put it in, but once taped down (to the cheek) and not moving, they can't feel it.
When my son had it done, the end of the tube was attached to an ipod size box which recorded the pH levels in his oesophagus over 24 hours. I'm not sure if now (5 or so years on) this would be even smaller.
If it reassures you at all, I have a doctor friend who said during their training, they had to practise putting NG tubes down their own nostrils and throats, and it is apparently just a funny / slightly unpleasant feeling, rather than painful. (This helped me when we were reinserting NG tubes the whole time when DS was little. I hated it.)
Good luck. I hope it goes smoothly for your DD. My DS was found to have severe reflux but we had very good results with Omeprazole.