It can be a weird post viral symptom! I think it’s more common with HFM but ours ended up with his nails peeling after he had Slapped Cheek
Yeah, my daughter likes climbing & throwing herself around rough & tumble play & her nails get slightly banged up in the process.
How is their calcium intake? Milk, yogurt, cheese? Lots of mums are skipping milk these days, I still find it essential for tots coz of the calcium. My boys’ nails never split as described. But he’s big on milk and cheese and yogurt (calcium is great for teeth and nail health)
It might be a vitamin deficiency?