Assuming your goal of turning your 18-month-old nephew into a genius is tongue-in-cheek, and that you just want to lay some groundwork for numeracy, I'd say add

Assuming your goal of turning your 18-month-old nephew into a genius is tongue-in-cheek, and that you just want to lay some groundwork for numeracy, I'd say add in some number words in your play together. Make trades, make up puzzles and if they aren't fun don't sweat it, move on. Be a caring, attentive, curious, kind, and intellectually humble adult in this child's life.