Child seats allow you to interact with your child and make them feel like they’re part of the magic of the bike ride. While trailers are handy for some, they can sometimes cause dirt or mud to be splattered on your child (especially so if it’s a rainy day!) and thus they don’t work for everyone. Trailers also take up a lot more of the road and can thus be precarious on bigger roads that are heavy with traffic.
Of course, trailers have some great attributes. The centre of gravity is nearer to the ground and thus if the bike falls over, they won’t fall so far, and they can often allow you to carry more than one child (perfect if you’re the only rider and have 2 children).
Ultimately it’s a personal choice that will be dependent on where you are opting to ride your bike, how old your child is, and how many children you will be lifting.