Parents think they have to have an answer to every spiritual question their kids ask. If you don't know the answer, find it together. Other than the Holy Spirit, nothing will solidify your young teenager's confidence in her faith better than searching for answers with you. At the same time, don't be afraid to leave some things unanswered. Part of knowing God is knowing that his ways are often beyond our understanding.
Experience is key.
Find ministries within the church where they can be involved in a meaningful way. Take your family on a short-term mission project. Getting kids out of their comfort zone has enormous implications for spiritual growth. If you're involved in some kind of ministry, involve your kids, too. I'd also encourage you to allow your young teens to respectfully question your values.