thank you for taking the time to write this.First NOTE TO SELF: you really do need to speak to an attorney from your state and ask them this. Each state is different, but most states do not have limitations like a parent not taking a child out of state without the other parent's permission. In fact, it is a really common misconception that you always need your STBX's permission to take your child out of state. This isn't necessarily the case. It depends on the status of your marriage...which at this time, you are legally married--no separation or divorce is filed.
Second, if your state's child custody laws are silent – they don’t say anything one way or the other about taking your child out of state without the other parent's permission – you're generally still prevented from doing so if your court order or parenting agreement says that you'll only do it with your ex's knowledge and consent. But you guys don't have a child custody plan or court order...so there's nothing that really says she can't go out of state with your child.
If you have no written order or parenting plan agreement in place, one parent is generally free to take your child out of state for short periods of time as long as it doesn't interfere with the regular custody schedule you've established (for example, to go visit relatives out of state for vacation). But keep in mind that there's a difference between whether someone can do something and whether they should. If she's planning to file for separation or divorce, she would be doing more harm than good to just try to walk away with the child--she'd harm her own case for custody--BUT you'd be facing finding her and getting your child back home, etc.
Thus, there's really nothing at this moment that's stopping her, but if you are afraid that she will try to take him, then take action and your state's equivalent of a temporary protective order. Even though you two haven't yet started your separation/divorce, get a temporary order specifying your custody rights. Establish limits for each of you on how far the children can be taken away from their home. For example, a simple order that the children are not to be taken out of the county in which they live without first getting written consent or a court order, will give you a lot of protection.