Sign language is not just for children who are deaf or hearing impaired. It has a number of benefits for young children. It helps them communicate their wants and needs in a non-verbal way and promotes social skills.
Studies have shown long-term cognitive benefits, including:

•  Accelerated speech and emotional development
• Increased communication skills
• Lower frustration levels
• Improved bonding between child and caregiver
• Larger vocabulary
• Improved literacy and promotes earlier reading
• Longer attention span
• Greater memory retention and word recollection
• Enhanced cognitive skills
• Higher cultural awareness

Further Resources:

https://www.cceionline.com/the-benefits-of-sign-language-for-preschoolers/

http://www.globalpartnership.org/blog/building-global-momentum-introduce-sign-language-early-childhood-education

https://www.canr.msu.edu/news/baby_sign_language_a_helpful_communication_tool

https://www.mamanatural.com/baby-sign-language/