I'm in secondary and you're generally expected to be onsite for the full school day, including PPAs. The only exception would be if you were 'blank' at the start or end of a day. A 'blank' is an unpaid period.
While I know the obvious answer is 'in the evenings', your school might wonder 'when will you plan?' if you suggest your PPA time should be at home for childcare reasons.
As a general rule, all PPA happens on-site, as we have room. Currently, we're allowed to take it at home if we prefer not to sit too close to anyone at school. I don't think childcare could or should be a reason for you going home, to be honest. Or at least, if it IS the reason, you'd probably best keep it quiet.