£70 a week isnt a lot but when you have toddlers its doable in that they are happy with a trip to the park, sleep in the afternoon etc. clothes are cheap and hand me downs are more aplenty.
However once they get to school age you have school dinners, school trips, school uniform, school shoes, pe shoes and kit, more parties etc, food costs increase as they eat more ie a toddler will have 1 slice of bread as a sandwich, a child will have 2 slices - instantly doubling your bread consumption - times that by every other food item and your weekly food shop is suddenly a lot more!!
then theres extra curricular activies, swimming/cubs/football etc all bring weekly charges and costs of kit and necessary items.
Days out suddenly cost more as children no longer get in free as toddlers, holidays cost more, etc etc
Even as toddlers theres unexpected costs - car tyre puncture, washing machine breaks, tooth filling needed, passport renewal, your shoes wearing out and needing replacing, child loses thier coat, birthday party, christmas costs, - all things that need to come out of your £70 a week!
its doable and plenty of people have to live nad to mouth but I wouldnt want to live on £70 a week if I didnt have to, its not poor by global standards but I wouldnt think most people in the UK would be able to have a full and fun life with a family of 4 on £70 a week.