I know it's informal but I also know little by little I'm introducing Math to my 18 month-old daughter by teaching her how to count with her hands, count toy blocks and let her be familiarize with numbers using flash cards and those puzzle mats with numbers on it. But if you want to take it seriously (not that I'm not serious too haha), probably more in-depth in enhancing their skills on that, here's a good read for you: https://www.zerotothree.org/resources/299-help-your-child-develop-early-math-skills