I believe I am in good position to speak since I am a BB officer

In my school, we make known to the parents that we are a
Christian-based CCA and that their child will be exposed to
Christianity. We do this on the day of recruitment. We also invite
the parents to the enrolment service. We make no attempts to hide
our Christian beliefs. We do invite them to attend church service,
and our cirruculum does involve Christian education. But our
activities also revolve around other areas such as leadership
training, community work, adventure skills, etc.
The MOE knows the nature of BB. I suppose schools should have a
good knowledge of what the CCAs do (If not I wonder what the
principals are doing).
At any time, the child can choose to stop his participation. If the
parents feels uncomfortable, then they may ask their child to do
so.
A bit of background. BB is the first uniformed unit for Boys. Sir
William Alexander Smith, founder of the BB, intended the BB to
place where Christian values can be imparted. Our Object is "The
advancement of Christ's kingdom among Boys and the promotion of
habits of Obedience, Reverence, Discipline, Self-respect and all
that tends towards a true Christian manliness".
The Scout movement branched out from the BB. Badon Powell was a
member of the BB. The Scout association was formed with not
religious background. So it's another alternative to a uniformed
group to choose from if the parents wants .