Let's focus on one part of your question as per Enotes policy: let's focus on interventions Early childhood special education, as mandated by idea, begins as early as age 3, depending when the disability is identified. The interventions are decided by a group made of the parents, counselor, principal, a regular ed teacher and a special ed teacher (depending on whether the district co-teacher), and the diagnoses of the psychologist. A resource teacher for learning impaired may be present if such is the case. All depends on the disability.
There is a list of interventions that are both determined by trained teachers, and by those compiled by the National Association of Special Education teachers. The list of interventions is given to the teachers to follow, as long as it is agreed upon and documented on the IEP. This IEP will say what the intervention is where it is to be conducted, who is going to provide it, and at what rate.
Interventions include assistive technology, behavior/consequences, shadow teacher, co-teaching, partial mainstreaming, language and communication skills, academic integration and behavioral control skills.
The main benefit of these interventions is the one on one support that the student gets from a free, non-threatening, and quality education as IDEA mandates.
The challenges of these interventions lay upon teacher training, funding, following up with the interventions list, quantifying gains or loses, and lack of documentation as of whether the interventions actually work or not. Parental support is also a challenge; if they do not enforce what is learned in school, the skils become weakened.