We use Sitters a lot at the moment - children are 10 and 8, so old enough to manage babysitters they don't know well, and we've been trying to get a social life back. The people we've had as babysitters have been universally great, and the site is super-efficient (almost too so, sequences of text messages confirming and re-confirming bookings). I am really happy with them.
Posting here because they also do ad hoc daytime childcarers too. We used them once when dd was new and I just had to have an extra hand with ds (who was 18m), and that was fine too - but there of course I was around, so it was less of a big trust issue.
Anyway, worth seeing if they operate in your area, franch?
The other thing is that workplaces often have ad hoc arrangements with local nurseries for emergency cover - mine does, and did for years before I found out about it - it wasn't publicised. My two are too old for that now, but we certainly made use of it a few times over the years (nanny off ill, etc.)