I get a weekly Tesco delivery and do a top up shop in Lidl mid week.
Eggs, bread flour, nuts, seeds, frozen salmon, frozen veg (sweetcorn and peas), canned mackerel, canned sweetcorn, chilled pizza bases, yoghurt, halloumi, feta, ice cream, biscuits, crisps, tea bags, Italian week espresso coffee beans, fresh pastries and bread are all cheaper and good quality in Lidl. For kids I get fruit pouches, wipes, bear yoyo type snacks (although I haven’t seen them in store lately) and rice cakes. These are also all good and cheaper than Tesco. The nappies are ok, but not as good as pampers imo. They’re much cheaper so I use them during the day when I change them often, but not at night.
Milk, butter and pasta are the same price in both supermarkets, so I usually get them in my Tesco delivery for convenience.
The fresh fruit and veg are not great in Lidl. Often you find some that are already bad in a bag or punnet. They don’t last very long and often are not tasty (blueberries or mangoes are sour, but if you keep them more than a couple of days they just go off for example). Their ‘green’ bananas are already ripe and then go black very fast, although DH still eats them, so I buy them occasionally.