Riser bars
Use riser bars to improve safety by mounting them under the treads as a barrier to close the gaps. This will prevent children from getting stuck and harmed. The building regulations (England and Wales) state that these should be fitted to all open tread stairs, particularly where children under 5 may be using the staircase.
The riser bars are specifically designed for spiral staircases Calgary, Keppa and Montréal 120cm / 140cm.
The bars are made of 10mm powder coated steel available in grey or white. In the package you get everything you need to install it right away – riser bars for 12 treads and chipboard screws.
Riser bars are designed to fit horizontally between the treads of an open-tread staircase. They reduce the gap between treads to less than 100mm, so that they comply with the 100mm rule in building regs.
https://www.laddersandaccess.co.uk/product/dolle-spiral-staircase-calgary-white-grey/