Summon a baby zombie riding a chicken:
`/summon chicken ~ ~ ~ {Passengers:[{id:”zombie”,IsBaby:1b}]}`
For a chicken jockey with a baby zombie in older Java versions:
`/summon zombie ~ ~ ~ {IsBaby:1b,Passengers:[{id:”chicken”}]}`
For Bedrock Edition, use a spawn egg or commands if supported by your version:
`/summon chicken`
`/summon zombie`
Use a spawn egg on a chicken only if your version allows jockey-style spawning via natural mechanics