presents to grandchild and now new step grandchildren

If you have a good relationship, could you discuss with the parents? I am aware of families where well meaning stepGPs treat all the children the same, but the result can be unfair.
The exact circumstances that lead to being a stepGP can be so different, I'm sure in a loving family you can come to a sensible arrangement.

thanks to all.
Very positive remarks.
Becoming a step grandma is very exiting but can be daunting at first.

It’s so refreshing to hear that you get on well with your step grandchildren. Ofcourse, it’s your money, your business. The natural grandparents should be pleased that the children have someone else, who loves them. You are doing the right thing by treating everyone equally. My grandchildren have a step grandma (my Ex’s new wife). I am almost sure she doesn’t treat my GC the same as she treats her blood GC. Ofcourse, they see her more often. And I suppose it’s only natural. Life !

We treat our stepdaughter the same as the other two in respect of pocket money for holidays, paying part of school trips, Christmas and birthday presents.

If the natural grandparents don't like it - tough !! Although I imagine they would be only too pleased that their GC are not being sidelined by their new GPs. By treating all the children equally you are doing the correct thing- it what I would do. Well done you !

My only concern would be if the step grandchildren already have the full quota of natural grandparents who might not be happy about this

I would ask first, making it clear that you regard them as your grandchildren and you do this for the others. I think you sound absolutely lovely