Infans.

It depends on the gender of the baby in question. 'infans' can be taken as a masculine or feminine noun, depending on the sex of the baby. It is declined the same way in both cases, so the use of 'infans' is correct regardless. However, the adjective 'economicus' must agree with the gender of the noun it modifies, in this case, 'infans'. Therefore, if the baby is male we have 'infans economicus' but in the case of a female baby we have 'infans economica'.

Presumably you are using it in a general case, not specific to a certain child, so the male form 'infans economicus' is fine.

It's 'infans'.

Infans

Infans