Things that come to mind:
Insurance, in case things go wrong - for equipment, for volunteers, and for public liability
Policies on equality and accessibility
Health and safety - training and assessments and record-keeping for all of this, including fire risks, trip hazards, etc
Data protection requirements if you're storing any personal details at all - names, addresses, dates of birth
The venue may have some or all of this, but you'd need to check with them exactly what's what. Most venues need you to agree all sorts of basic safety and insurance things with them before letting you use their space these days.
You'll also need to be able to consider whether it's ASD-friendly. Many venues aren't: noisy, echoey, flickering fluorescent lighting, no way to secure the doors to stop the runners, etc.
Equipment - how will you entertain them all for that amount of time, or will the parents need to bring their own stuff?
Then there's the challenge of all the different points of view and having to mediate between them.
Brilliant idea, but having co-run a toddler group for some time, not that easy. Worthwhile if you can get a bit of help and interest, definitely.
Will the NAS help you in some practical way too?