Enrolling your child in a Spanish speaking preschool might be the easiest way for them to pick up the language at a young and versatile age.
Preschool education became mandatory in Mexico in 2004, but there is still an astounding amount of children who are not enrolled. Around 60% of children aged 3 do not attend preschool.
That being said, there are many preschools to choose from. Some schools offer K-12, which means that they offer schooling from the age of 3 to graduation at the age of 18. Public preschools are free and mandatory. Private preschool tuition fees will vary, but they are reasonably affordable ($2,000-$5,000 MXN a month). You can find a private kindergarten here.