M.C.
With my kids I try to go in the late afternoon - 3pm or later. This way the sand isn't as hot to walk on. You would still need an umbrella for shade, but it would be more comfortable, and you wouldn't have to adjust the umbrella as much as the sun moved. You can also put baby in a light sleeper, that way only the face/arms would need sunblock. Also, don't forget to take a sun hat.
Is going to the beach with kids those ages doable? Yes. It is good that you have a 1-1 ratio.
I haven't heard of that type of sunscreen. Maybe as a pharmacist or your pediatrician for a recommendation?