Had both a tag along and a kiddyback tandem (and both together!). The tandem is a bigger initial expense but FAR superior. Seat tube mounted tag-alongs can exert a significant wagging effect. I had a Burley that mounts to a specific rack, and this is again, far superior (and a lot more expensive). So in order of decending price/utility
1) Kiddyback tandem (from 4 will be fine)
2) Rack mounted tagalong, gears are helpful (Burley and an old Isla)
3) Seat-tube mounted tagalong.
None will teach a child to balance a bike. You will be doing that for her. She’s also a little young to learn gears, a tandem gets over that.
For reference, my then 6yo son rode London-Oxford on the tandem, he would never have gone that distance on a tagalong.