No. Children would not have had the perfect use of reason from the very outset because in all things produced by generation nature proceeds from the imperfect to the perfect.
Pueri non statim a principio habuissent perfectum usum rationis quia natura procedit ab imperfecto ad perfectum in omnibus generatis.
Even other animals have not at birth such a perfect use of their natural powers as they have later on. This is clear from the fact that birds teach their young to fly; and the like may be observed in other animals.
Etiam alia animalia non habent ita perfectum usum industriae naturalis statim a principio, sicut postea. Quod ex hoc patet, quod aves docent volare pullos suos, et similia in aliis generibus animalium inveniuntur.
The use of reason depends in a certain manner on the use of the sensitive powers; wherefore, while the senses are tired and the interior sensitive powers hampered, man has not the perfect use of reason, as we see in those who are asleep or delirious.
Usus rationis dependet quodammodo ex usu virium sensitivarum, unde ligato sensu, et impeditis interioribus viribus sensitivis, homo perfectum usum rationis non habet, ut patet in dormientibus et phreneticis.
Now the sensitive powers are situated in corporeal organs; and therefore, so long as the latter are hindered, the action of the former is of necessity hindered also; and likewise, consequently, the use of reason.
Vires autem sensitivae sunt virtutes quaedam corporalium organorum, et ideo, impeditis earum organis, necesse est quod earum actus impediantur, et per consequens rationis usus.
Now children are hindered in the use of these powers; wherefore they have perfect use neither of these powers nor of reason. Therefore, in the state of innocence, children would not have had the perfect use of reason, which they would have enjoyed later on in life.
In pueris autem est impedimentum harum virium. Et ideo in eis non est perfectus usus rationis, sicut nec aliorum membrorum. Et ideo pueri in statu innocentiae non habuissent perfectum usum rationis, sicut habituri erant in perfecta aetate.
Yet they would have had a more perfect use than they have now, as to matters regarding that particular state, as explained above regarding the use of their limbs (Q99, A1).
Habuissent tamen perfectiorem quam nunc, quantum ad ea quae ad eos pertinebant quantum ad statum illum; sicut et de usu membrorum superius est dictum.
The corruptible body is a load upon the soul, because it hinders the use of reason even in those matters which belong to man at all ages.
Aggravatio additur ex corruptione corporis in hoc, quod usus rationis impeditur quantum ad ea etiam quae pertinent ad hominem secundum quamcumque aetatem.