tzo'ah of a child who was capable of eating a
kzayis of grain within the time of
achilas pras, but has not done so for several days or has lost this ability because of an illness, is also forbidden.
B'dieved, a person does not need to repeat
Shmone Esrei in this case, but he should repeat
tzo'ah of an older child (over the age of six/seven) is likewise forbidden. Although the halacha only mandates a distance from the
tzo'ah of a child capable of eating a
kzayis as above, it is recommended to avoid the
tzo'ah of any child once it is eight days (and possibly even one day) old. This is not the case with a child's urine.
ס"ק א-ג; ביאורים ומוספים דרשו, 5)