Whenever you do a command to spawn in a mob that has "IsBaby" tag:
/summon minecraft:zombie ~ ~ ~ {IsBaby:1}

It will spawn in the baby variant of that mob. 

Whenever you try it with Strider:
/summon minecraft:strider ~ ~ ~ {IsBaby:1}

It will not spawn in the baby variant of the Strider. 

The same goes for the Zoglin. 

/summon minecraft:zoglin ~ ~ ~ {IsBaby:1}