There are 40/2= <<40/2=20>>20 women at the event.
Hence, there are a total of 40 + 20 = <<40+20=60>>60 men and women at the event.
So, the total number of children at the event is 80 - 60 = <<80-60=20>>20.
After Oleg added 10 children to the guest list, there are a total of 20 + 10 = <<20+10=30>>30 children.
#### 30