Not feeling able to push your grandchildren in their pram, due to the tics, must be upsetting. However, I think that it should be possible to design an adaptation for the pram to dampen the impact of your tics so that the children have a smooth and safe ride. This is the sort of design challenge that REMAP can often help with:
Also I suspect that you are not the first person to encounter this issue with pram pushing, it might be worth also posting the question on:
Jean Merrilees BSc MRCOT
You can read more of my posts at: https://community.scope.org.uk/categories/ask-an-occupational-therapist
I have just checked both the links again and they are working fine for me. Perhaps you will need to enter them into browser or use google to find the sites that I suggested: firstly REMAP and the other link ( https://www.disabledparent.org.uk/) goes to the website for an organisation called Disability, Pregnancy &Parenthood.