anyone knows where?

There are actually quite a few options depending on where you stay, Dr Dave Ong's clinic opens on select Sundays, and someone in my mum group mentioned his schedule works well for weekend sick visits, though you'd want to call ahead to confirm timing. Some children's clinics also open Sunday mornings (typically 9.30am to 1.30pm) so it's worth checking a few near your area since availability varies by location. If it's more urgent and you can't get a weekend appointment, the A&E at private hospitals can arrange a paediatrician on call, though that does come at a higher cost, so I'd say try the Sunday clinic route first, and keep A&E as a backup for anything more serious.