before your child can hold his or her head up for reasonable lengths of time, the best thing is a car seat secured to the bike or in a trailer. it goes without saying, i hope, that you will ride with extreme caution, taking it very easy over bumpy surfaces. lower tire pressure if required to smooth things out. go slow, avoid routes requiring interaction with higher-speed motor traffic, etc.

your local laws may require that the child wear a helmet. this makes no sense in the car-seat scenario, but it wouldn't be the first time you must decide between legal and sensible.

somewhere around 9 or 10 months a front-mount seat is ideal (google "bobike mini" or "ibert") for short rides. many babies fall asleep quickly on bikes, so keeping them stimulated with chatter and supportive touch is important, and having them within the circle of your arms is ideal.

most kids outgrow the front seats around age 2. move to rear seat. bobike maxi brand is compatible with xtracycles 95% of the time.

trailer or bakfiets with cover is ideal for longer rides where the child may sleep, eat, play with toys, or require more complete weather protection than a seat makes easy.

around age 5 (you'll know when) your child can probably sit on the back of an xtracycle without a dedicated seat. bobike jr. model seat is rated up to near 80lbs, too.