What I would do is- I would affirm her desire and heart to serve, but would explain that one of the prerequisites to serve in kids is to be a believer and accept Jesus as Lord. If there are other places she can serve, I would point her to those areas and get her connected. If you have the capacity, I would offer to meet with her regularly and walk through discipleship/bible study with her (or find someone who can meet with her regularly). The “no” isn’t forever, just “not yet” while she works through her beliefs and faith.