I would suggest planning activities to do with them on separate occasions. Speak to them about your plan and let them know that you will do something with each child on their own.
Speak to your children and find out what they are interested in and where they would like to go to have some mum and child time. I would even pop this into the calendar as a reminder so they know what to expect. When spending time all together - plan a fun and fair game or choose a movie that you can all watch together.