I was a youth worker before having children, but was supported by an inter-denominational network of churches rather than one individual church.
I loved it - I was meant to be part time at first, but it doesn't ever work out like that - lots of evenings, weekends and early morning starts for assemblies etc. I ran a lot of after school clubs too.
If you're only dealing with one primary school then it'll be much more manageable. They will want you to be CRB checked/disclosed through them even if your church has already done so.
Building a good relationship with firstly the head and then the secretary are key. I found that signing in and out (sounds insignificant) was actually a vital part of keeping the doors open.
You could offer a lunchtime club, assemblies, after school club and open invites to whatever childrens events are happening at your church. Some schools may ask that children attending clubs, even if they're voluntary, must get a permission slip from their parents. This may be a long and drawn out process, but it's more important to be accommodating to how the school does things. Being honest, transparent and up front about what you're in the school for is also vital.
Otherwise there'll be the inevitable accusations of indoctrination etc.
Keep the stuff in school low key and if children/parents want to respond further, then that's where your link with the church comes in.
You'll be there to sow seeds and not much else
You will also need a handful of people in your church to act as a steering group of some sort - to support you, pray for you in your work and do practical things. You may need to apply for funding/insurance etc etc - you're the worker at the coalface so to speak and this is not your responsibility.
I'm sure I'll think of other stuff, but hope some of that helps?