you can spawn them just like the normal chicken jockeys by using spawn eggs (1/2000 chance afaik), but the baby zombie pigmen does not control the chicken. The chicken AI still works (seeds ..). If you anger the pigmen it will only be able to attack if you're in close range.

You can try it yourself by using:

'/summon PigZombie ~ ~ ~ {IsBaby:1, Riding:{id:Chicken}}'