I have flown for 32 years on both long and short haul with 3 kids.
During this time i have had a variety of childcare solutions.
If you are full time with little family support the best option is a live in trained Nanny, who has your days off.
If you are part time, or have some family support, you can put your Nanny/childminder on an annual hours/days contract---giving her a monthly salary based on the hours/days you think you will need her.
I never bothered with agencies, but advertised myself in The Lady, Nursery World and my local papers.
Never failed to find the right person, but it does take time and hard work.
Best of luck!