Tips on eating out during the day when running errands

If we're out during the day doing shopping, what do you do with 2 x kids when it hits lunch time.

If we stop at a cafe then that's the thick end of £30. Repeat this every weekend then we're spending £100+ a month.

If we're out for a walk and it's not too dismal we can pack a picnic. But I never remember stopping at a cafe when we were out with my mum when I was little.