Fontanels are the soft spot found on the top of the skull of puppies at birth. This area is present due to the incomplete fusion of the bones that grow together with more maturity. Many toy breeds will not be completely closed by 8 weeks. This is NOT abnormal unless extremely large. If they are large, they pose a brain injury risk if the pup gets hit in the area. The open fonts are GRADED by brokers as .1cm ( size of a pin head), .5mm (about 2x the size of this 0), and 10mm (the size of a small blueberry).
Feeding calcium will close them faster. The only time I worry about open fonts is if the pup very tiny (not thriving) and WALL EYED (eyes protrude out) too. This indicates hydrocephalus (water on the brain) and the pup will seizure and die eventually. This is very rare. There is no reason a puppy that once had an open font in infancy cannot be safely bred.