So it looks like a nursery job to me... so you would be after someone with Level 3 suitable qualification, able to pass Enhanced CRB check, suitable experience, knowledge of EYFS etc.
Those people will in my view either be:
Young having just finished a childcare course at college.
Young having trained on the job in another childcare setting.
Middle aged singleton (like me)
A nursery nurse who is now a parent themselves so may have young children and or school aged children.
Some very experienced with childcare... their own children now having left the nest.
Anyone else it may appeal to?
Young people may like it as it does not require them to get up early. However are they looking for part-time work, or are they needing 40 hours+?
Middleaged singleton like me would need more hours, as I have a mortgage to pay... doubt working 30 hours a week would be sufficient.
Nursery nurses with their own young children may find that most of their salary is used to pay for childcare for their own children (even with you offering it at reduced rate). This is a common issue for many working parents... is it worth working given the cost of childcare.
Why do you operate 8am-6pm? Is that where the demand is or given that you mention late collectors are parents needing you to open till later? May be worth sending a survey out to users to see what the ideal hours of operation would be. You will need to look at costs of course, given that you will need two members of staff at all times as a minimum (excluding management) to meet Ofsted requirements (I am assuming you are in England, clearly if you are not then different rules may apply).
there is nowhere to have lunch (other than sitting in their car)
That I see as being a problem, regardless of your issue covering shifts. Your staff can't be very happy about not having anywhere to go on their break. I've known managers offices to double as staff rooms - not ideal though better than sitting in the car. Consider if another location can be provided - such as a Summerhouse in the grounds, not ideal but it usually won't need planning permission.
Free meal for them
Free childcare for school age children
Subsidised childcare for under 5's
31 days paid leave (inc bank holidays)
Uniform
Not sure all of these are perks... given the arrangements for lunch - I presume the staff eat with the children, which actually works well in nurseries - lunch being provided I would see as being essential. If the children see the staff eating the same as them, they are more likely to try eating it in my view. Also the staff can identify any issues with meals, as they are serving it and eating it.
Free childcare for school age children could help encourage someone to take the shift, so yes that is a perk.
Subsidised childcare - see my point earlier. May or may not be that viable for the staff member... or for you... as having staff and their own young children in the same room can be problematic. I've known nursery staff and nursery managers to have their own under 5's in nursery and it does sort of work but the child will always go to mummy, demand mummy when they are upset about something.
Not sure why you are offering 31 days holiday... it's above statutory minimum and must cause you some timetable issues. Is that a perk staff like? There's a topic for the next staff newsletter.
Uniform - that's not a perk... it comes as part of the job. For practical reasons and aesthetic/branding reasons you will want your staff to be wearing a uniform.
Would being paid until 6.30pm but going home early sometimes be a perk which staff may like? Maybe something to offer on the graveyard shift.
Not sure I'm helping at all. Probably giving you things to think about which you don't want to think about, rather than resolve your current issue.
Have you got any long term members of staff? Have you asked them about what they feel about working each of the various shifts? They may come up with reasons why they like their current shift and reasons why they wouldn't like the other shift or a change in the hours of their current shift.
Maybe useful to know your location and more info about your setting. Could you send me an e-mail (see my Mumsnet Profile)... if in England, the URN would be handy as that will tell me quite a bit
Have you plotted on a map where your current staff live? Visualising things can sometimes help identify target areas where you may want to have recruitment advertising. Companies (even small ones) often use information mapping systems - even a basic pin map, I've used on of those before when working for a small company - to plot where customers are located... so I expect you may already do mapping for where the children live, thus areas where advertising could work well.
What has happened to the people who have worked the 12-6 shift in the past? Have they given any comments as to finding the shift hours difficult?