Our divorce agreement is very open-ended on this question.
But does it have to be so open ended? I know a lot depends upon the laws in the state/province where you live. However in my state, my L was able to add legalese that says that the person with primary physical custody has the final say in decisions. Ask your L about something like that.
But, do you have primary physical custody or do you split 50/50? If you have primary then ask your L to add language that says that DD will go to public school in the district where the primary parent lives.
Another thing to consider is whether you school district offers pre-K and kindergarten. If they do then THAT is where your focus needs to be, not some preschool that your X wants to send DD. You can simply say "No. I am happy with DD's current care."
Finally, educate yourself on all of the school zones in question. Not just elementary but also the middle schools and high school. You want to pick the best school zone, for all 3, that you can afford. Your DD is a young, a toddler? She will not remember who her "friends" were in preschool. So separate that from everything else. Right now, at her age, YOUR support system is more important.
Consult your L and do what makes sense for YOU. If it is "hard" for her", then so be it. She chose the action, she needs to deal with the consequence.