There are many offering of child bike seat available for kids under 3 years old, but offerings are much reduced after that.
As with all load carrying on a bike there flat ground is required, if you want to cycle reasonable distances. A bike ride carrying a 10kg child, is 5% more difficult on flat ground and 50% more difficult on hilly ground.
Child bike seats also make you bike more top heavy particularly when you are pushing the bike. Without your own weight on the bike, a very large percentage of the bikes weight is set high up, and the bike can flip if it goes a few degrees past vertical.
Enough doom and gloom. I have cycled 100’s of miles with no drama with a child in a bicycle seat. Some of my bikes will carry 3 kids. Although I have not done this very often effective range is about 3 miles!