If my wife and I are separated, but legally, can I force counseling before we contemplate a divorce?

Your question indicates that you live in Texas... so I'm going to give you a Texas perspective and some multi-state issues to consider.
Texas does not have legal separations.  Either you are married or you're not.  Right now you are still married.  This means that both you have the same and equal independent rights to the children and any property.  If you go to the marital house, she can't stop you from entering.  If it's between you and her, and she was first in possession, the cops won't usually require her to release the kiddos to you to visit.  However, if the kids are at school, then you can go visit or get them for a period of possession--because your right of access is superior to a third party's.  This will be the situation until some type of orders are entered.
With that in mind.... you can file for a divorce without finalizing a divorce action.  So... you may want to consider filing for divorce so that you can get some temporary orders in place.  Temporary orders will serve as the official rulebook while you and her decide if you want to move forward or not.  You can request counseling, mediation, and possession/access via the temporary orders.  If you and her decide not to follow through with the divorce, you can always dismiss the divorce action if you and her reconcile.
You mention that she's in another state.  Texas has a six month residency requirement before you can file for divorce in Texas.  If there is a possibility that a divorce can or will happen, you may want to file now versus later.  Not all states, but many other states have similar residency requirements.  This means that it will be even more expensive and difficult for you to defend yourself against a divorce action if she establishes residency in her state and files first.  Whoever files first usually gets the right to finish the divorce action... and the second filed case in the other state will get dismissed. 
Because you do have multi-state issues, it would be worth the effort to consult with a family law attorney in each state.  You need to know what your exposure is in both states.  In Texas for example, you are only responsible for child support until the kids graduate from highschool or turn 18, whichever occurs later.  In Missouri, your support obligation could extend all the way through college.  You also need to know the rules for division of property.  Texas is considered a community property state... but many states are separate property states.  This can affect who gets what in the divorce.  Once you have an opinion from each jurisdiction, you can make a decision on what to do and how best to proceed.  More and more family law attorneys are offering inexpensive consultations... it's a short term investment in the potential long term impact of your marital issues.