Why 2 years old boy finger nails always crack?

Rash? This can be associated with eczema or psoriasis, or rarer skin conditions. Are there any signs of skin problems any where else on the body? If there are no othe skin problems this can be associated with certain deficiencies such as zinc. See pediatrician for exam and approriate blood tests. Topical treament which can help if there is no underlying issue is dermanial.