The resources we use for 0-5 are "Bubbles" by Scripture Union: loads of ideas and a weekly programme.
A few things to consider:
Location - you need an indestructible, safe room with space and toilet access.
Leaders. Ideally need 2 leaders each week . This is always a challenge! Preferably CRB checked.
Activities. It's worth having a "toybox" for the tiny ones as they obviously won't be ready for colouring-in or story activities, although might enjoy watching. Get the parents to contribute some age appropriate toys, books, jigsaws (need not be religious).
Then you can do a few activities for the 3-7 children, maybe games, singing, story, craft activity (this is where the Scripture Union material comes in). Drink and biscuit break too.
If you are a mainstream denomination they might have a local co-ordinator for childrens work who could advise you.
Any questions do come back.
It sounds like quite an undertaking but in these days you do have to be serious