All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah.
First: Seeing Angels
Seeing angels in forms other than their true appearance is possible. It is established that some of the Companions saw them in human form, as in the hadith of Jibril (Gabriel), peace be upon him, when he came to the Prophet ﷺ in the form of a man, and the hadith of Sa'd ibn Abi Waqqas, may Allah be pleased with him, concerning his seeing two angels fighting alongside the Prophet ﷺ on the day of Uhud.
As for seeing angels in their true forms, this was established for the Prophet ﷺ when he saw Jibril, peace be upon him.
Second: Seeing Jinn
Seeing jinn in forms other than their true appearance is also possible. It is established in Sahih Muslim that Abu Hurayrah, may Allah be pleased with him, saw the devil who used to come to him while he was guarding the Ramadan charity. The Prophet ﷺ informed him that it was a devil.
Third: Can Infants See Angels and Jinn?
We know of no Islamic text establishing that infants can see angels or jinn, just as there is no evidence denying this possibility.
Accordingly, an infant's laughter or looking at an empty space is not evidence that the child has seen an angel or a jinni. These matters belong to the unseen, and it is not permissible to make definite claims about them without evidence from the Quran or Sunnah.
Conclusion: Seeing angels and jinn in forms other than their true appearances is possible. However, there is no established Islamic evidence that specifically states that infants can see them. It is therefore incorrect to interpret a baby's laughter or gaze into empty space as proof that the baby has seen an angel.
And Allah knows best