On hot, sunny days, your babies are going to be exposed to harmful sun rays even if you stay in the shade - make sure they are well protected with sun cream or sun block, but make sure that the products you apply are suitable for babies.
Sun creams work in one of two ways. Chemical creams bond with the skin to create a protective layer from the harmful ultra violet rays from the sun. Physical 'block' creams form a surface over the skin and, although mostly invisible, preventing harmful rays from reaching the skin.
Make sure you use blocking creams for babies. These are mineral based, usually listing titanium or zinc as their active ingredients, and characteristically don't rub in so well, often even leaving a faint glaze over the body. This may not look great, but is good news as you can see the sunblock in effect.
Sun creams marketed at children should be the physical sun block type so it is worth seeking out those products specifically, even if they are more expensive than adult sun creams and even if they don't appear to 'rub in' well.
There is also some concern that chemical based sun creams may be carcinogenic as tests on animals have shown that some ingredients cause cancer cells to multiply more quickly and have also led to developmental problems. Perhaps it's best that you also started using the kids' suncream just to be sure...