i found out a couple of weeks ago that if you ask at the customer service desk they will give you a 'toddler trolley' ie a trolley with a double seat so you can sit two children in it.
apparently they're not outside with all the other trolleys cos 'they walk'.
a notice saying they're available might be useful!
i post this after months of struggling with a single seat trolley and two children - taking turns, one in trolley trampling shopping etc so hope it's not stating the bleedin' obvious!