You want your child to become bilingual. You don't want it to cost a fortune and don't want it to be a big stress.
Here is the path you can take:
1. Start with Spanish for You! in elementary or middle school. It is inexpensive, easy, and builds solid foundations in Spanish. Starting early takes advantage of your child's "brain" ability to learn languages more easily and become savvy linguists later.
2. Take 4 years of high school Spanish. Since your child has already been learning Spanish, he/she will excel with confidence at this higher level.
3. Take Spanish classes in college - conversation classes when possible. If your child does not go to college, then encourage some conversation or literature classes at the local community college.
4. Do a study abroad in a Spanish speaking country. This is the "icing on the cake". The student is now highly challenged to take all he/she has learned and push it to a fluency level. I recommend a minimum of 5-6 months where Spanish will be spoken every day.
That is it! It takes years, but is doable for just about anyone. The biggest expense comes with the study abroad. At that point, the money investment is worth it.
If you have any questions about getting your child started learning Spanish, feel free to contact me at firstname.lastname@example.org.