Thanks again, everyone for the great suggestions.
I hope to try Ann's idea early next week when I work with the group, I think it will prove really helpful.
I thought lots about Di's idea of leaving the children something to hold onto to help them feel safer when I'm not around, and kept my eye out for some little soft toys but didn't find anything suitable.
This morning, I was gathering some resources at home ready for school and stumbled across a pack of four brightly coloured pieces of soft rope I've had for ages, and had an idea.
I cut the ropes into short 6" sections, tied the ends so they won't fray and knotted a loop in the middle. I let the children each choose one and explained for the times I am out or not available, they can use the little sections of rope. We talked about how it is really important they can use their network if they need to talk to someone or if they feel unsafe and I'm not there. Also, the loop on the rope can remind them even when I am not there, I will always be back again (later/next day/Monday), like a loop.
Hope it works!
judith