Well, YANBU to want Tesco to have more double trolleys, and YANBU to put a complaint/ suggestion in writing and e-mail it to the manager of your local store. But YABabitU to expect them to be there. Yes, it's a pain. Same way as lots of inconveniences in life are a pain.
I'm not going to judge you for your son running off- dd1 was a bolter, so I can sympathise. however, given that it happened, and I'm sure it was horrible, and you won't want it to happen again. And given that you cannot guarantee there will always be trolleys to suit you, I'm thinking your best bet is to listen to the advice of the online shoppers!
I have 3 (who were all under 4 at one point) and I soon gave up any idea of shopping in a supermarket- far far too stressful! So I do a combination of online (for the non-perishable items) and top-up shopping. Many butchers and fishmongers deliver- mine deliver to my work, the wonderful souls. And when I was a bit more flush, I also got an organic fruit and veg box left at my back door while I was out at work (sadly finances no longer allow!)
Online has been great for me. Yeah, I get the occasional odd substitution, or short use-by date, but compared to the stress of 3 children all mewling at me for things, all needing the toilet at 3 different stages of the shop, and then getting home to realise that in trying to distract them I have forgotten half the stuff on my list, it is a godsend! Give it another go, seriously!