What are the interventions to be used before and after an echocardiogram on an infant?

None. It's just a routine test. Before it is done, someone must have examined the patient and found some reason to do the test, like a heart murmur. After it is completed it would be interpreted and the results could lead to some form of treatment such as medication or surgery.
Check your lessons. This site is not a good shortcut to preparing your nursing homework. Your instructors will point out reading material on preparing patients for procedures and will expect your homework to reflect the jargon contained in these references. This site is for real patients who ask questions about their medical issues.
For transthoracic . There are generally none...The calming presence of a concerned parent is sufficient.