There are lots of internships and volunteer work that involve working with children - for example, research assistant positions in labs, tutoring/teaching, camp counseling, etc. Career-wise, you can go in a more academic direction by continuing to research children (as a professor, lab manager, graduate student, or researcher), or you can apply your work to any field involving children. For example, you could go to medical school and become a child psychiatrist or a pediatrician. You could become a teacher. You could do social welfare work or other work that advocates for the well-being of children. Almost any field has some applicability to children, and what you learn about child psychology and about doing research is likely to be applicable in some fashion.