While ever there are no orders in place, you are a little bit powerless to do anything, so it's important that you get something on paper to stop that from happening.
Your first step is to call the other parent to family dispute resolution to try and reach an agreement for the kids' care arrangements. You can do this through Legal Aid and either do it with or without the assistance of a solicitor. Hopefully, you will reach an agreement, but if not, or if your ex refuses to attend, you will at least receive a section 60(i) certificate, which you must have before commencing court proceedings.
If you do reach an agreement, you can have them made into consent orders so the court has some legal power of intervention in case one or the other breaches the orders.
I hope this helps. If FDR fails, feel free to revisit for some additional guidance.