when our two when similar ages to yours we had one big shop delivered a month from asda and topped it up by buying fresh stuff once a week.
Ordering online meant that we were able to take our time ordering stuff and making sure we were getting the best deals, plus by not going to the shop we weren't buying as much stuff that we didn't need by just picking it up and sticking it in the trolley.
Also meant we could buy in bulk i.e a full months worth of nappies as we didn't have to fit it all in the boot