Jesus sent forth his disciples with these words from Mark 16:15 "Go ye into all the world, and preach the gospel to every creature." As a homeschool mom, I place Bible above all other subjects because we are first called to disciple our children and then lead them into their academic studies. But it takes more than Bible study to cultivate a heart for missions.
H - Host a missionary. Open your home to a visiting missionary and let your children enjoy firsthand stories of God's work in another country.
E - Be an example. What you believe about the gospel, people, and missions must be visible. Show your children that missions are a priority by the way you spend your time, money, and talents. I am the treasurer/secretary to our missionary in Spain, so my children see me working weekly to prepare deposits and reports; and they are rewarded with a candy every time we make a trip to the bank. I also allow my children to help me when we have reports to mail, they help fold letters and stamp envelopes; and this is just one way we can all serve and further God's work in Spain.
A - Affinity. Build a personal relationship with a missionary. Connect with a missionary family that has children of similar ages. let your children become pen pals with the child closest to their age. This is a great way to keep up with current events and prayer needs in their part of the world.
R - Read the missionary letters. Read about the country they serve in and get to know the geography, culture, and people. But don't stop here. After reading, pray for the missionary and allow God to use you to help give to those in need.
T - Take a trip. Take a short mission trip close to home with your children - empower them to impact their world in ways that are relevant to them. My youngest daughter has been in the hospital so much and throughout all her stays, she has come home with many gifts. Our family wanted to bless others in return and now we take gifts to the children in the hospital at Christmas and we share the gospel by placing a tract with each and every gift. There is no better way for children to cultivate a heart for missions than to be a missionary!