Babies up to a few weeks old can easily get through and run through the grid holes in a C&C cage. In addition, some guinea pigs up to a few months old have thinner and pointier-shaped heads which can also lead to them getting their heads stuck in a grid hole. Please keep in mind we are referring to the standard grid sizes of 14" square with 8 by 8 inner wires, giving 9 holes by 9 holes across of just under 1.5 inch openings. Some grids on the market that people use to make their own C&C cages are the unsafe variety because while they are 14 inches across, they use fewer inner wires leaving much larger and unsafe openings. Please do use them at all.
Most importantly, with babies in C&C cages, do NOT use any grids INTERNALLY in the cage. No inner supporting grid walls, no hay racks made with grids or wires, no ramps fashioned from grids, etc.
Please pay attention to the SHAPE and SIZE of your guinea pigs' head, no matter how old. Again, if it's very thin and pointy head, consider it a baby and take appropriate precautions.
The great, vast majority of people who house babies in a C&C Cage are fine because the 6-inch walls are usually sufficient. However, you don't want to be the 'one' that has a tragic accident. There are several ways to baby-proof a cage until they've grown enough to move past the risk. BABIES GROW FAST! So, you really want a temporary solution to blocking off access.