Well, there would be two ways how to create this. If we're going to use trigger, this may be a complex one.
One easy way I thought of is with the use of Calculated Fields, but this would only be on a DataPage Level.
You can use this formula to get the Minimum Value.
SELECT MIN(FieldName) FROM Child WHERE ID = target.[@field:FieldID]
Assuming if you have a foreign Key inside your child table. So we'll be using this formula to display the minimum value, Datasource would be your parent table.
I hope this helps.