GX5G-X89 Lars Helsing and his wife Brita Lisa Karsdotter GHWZ-CRF
I have their 2 children, Karl and Marta, but what last name do I give them? Helsing or Larsson and Larsdotter?
Here is some good basic background: http://www.hhogman.se/naming-practice-sweden.htm
Since these children were being born in the 1880s, 1890s, and 1900s, there is a good chance they never used a patronymic surname. You best bet is to find a later record for the children and see what last name they were using.