From about 1 year of age the user’s three children have been able to sit freely on his lap without a harness. Before that age, they wore a harness, see related tip.
When the children were really small, they could be suspended in the baby carrier facing the user. This solution worked until they were about 8 months old and too large for their legs to fit. Once the children are old enough to sit by themselves, usually nothing is needed when the children ride along.
See also www.youtube.com/watch?v=dWZI_wFwZnE#t=101 (last checked March 2021).