Nested IF THEN ELSE can and will get very confusing and difficult if changes need be done.. also I find using Select case for conditions that have more than two options is better.

Break it down to steps and Later do all the calculations..

IE

Code:
Adults = Cint(txtAdult.Text) 
Child = CInt(txtChildLow.Text)

'Select the price for the day
Select case DayOfWeek
    case "Monday",  "Tuesday", "Wednesday", "Thursday"
        AdultPrice = 10
        ChildPrice = 8
    case "Friday"
        AdultPrice = 15
        ChildPrice = 8
    case "Saturday"
        AdultPrice = 20
        ChildPrice = 15
    case "Sunday"
        AdultPrice = 20
        ChildPrice = 10
End Select

'Calculate max free Child meals..

Freechild = Cint(Adults / 2)

'Work out paying child meals

If Freechild >= Child then
    PayChild = 0
Else
    PayChild = Child - Freechild
End If

'Now calculate final Price..

Total = Adults * Adult Price + Paychild * Child Price
So here I've shown you how to calculate According to the day of the week, you can aplly the same logic to use promotions...