It works wonderfully. You are my hero right now, sfdcfox

How would you most elegantly expand this case to cover a second type of children, say, foster children? 

Not every parent would necessarily have foster children, just as every parent wouldnt necessarily have regular children, so the parent map would have to be ALL the parents related to the grandparent instead and then only display children when they exist?

 This controller works. I'm sure there are ways to condense and clean this up but its pretty easy to understand this way. I'm filling in two trees for my two child types, incomes and contacts with the SAME parents and grandparent maps

The only maps that differ are the child maps

public with sharing class IntakeEXT {
	
	private Family__c family;
	public map<id,Participant_Income__c> incomes {get;set;}
	public map<id,Participant_Contact__c> contacts {get;set;}
	public map<id,Participant__c> participants {get;set;}
	public map<id,Family__c> families {get;set;}
    public map<id,map<id,id[]>> treeInc {get;set;}
    public map<id,map<id,id[]>> treeCon {get;set;}
	
	public IntakeEXT(ApexPages.StandardController controller){
		family = (Family__c)controller.getrecord();
		
		incomes = new map<id,Participant_Income__c>();
		contacts = new map<id,Participant_Contact__c>();
		participants = new map<id,Participant__c>();
		families = new map<id,Family__c>();
		treeInc = new map<id,map<id,id[]>>();
		treeCon = new map<id,map<id,id[]>>();
		
		for(Participant__c participant:[SELECT Id,Name,Family__c FROM Participant__c
							WHERE Family__c = :family.id]) {
			//populate parent and grandparent levels of Incomes tree					
			if(!treeInc.containsKey(participant.Family__c)) {
		       treeInc.put(participant.Family__c, new map<id,id[]>());
		    }
		    if(!treeInc.get(participant.Family__c).containsKey(participant.id)) {
		       treeInc.get(participant.Family__c).put(participant.id,new Id[0]);
		    }
		    //populate parent and grandparent levels of Contacts tree
		    if(!treeCon.containsKey(participant.Family__c)) {
		       treeCon.put(participant.Family__c, new map<id,id[]>());
		    }
		    if(!treeCon.get(participant.Family__c).containsKey(participant.id)) {
		       treeCon.get(participant.Family__c).put(participant.id,new Id[0]);
		    }
			families.put(participant.Family__c,null);
			participants.put(participant.id,null);
		}
		
		for(Participant_Income__c income:[SELECT Id,Name,Participant__c,Participant__r.Family__c
		                    FROM Participant_Income__c
		                    WHERE Participant__r.Family__c = :family.Id]) {
		    treeInc.get(income.Participant__r.Family__c).get(income.Participant__c).add(income.Id);
		    incomes.put(income.id,null);
		}
		
		for(Participant_Contact__c contact:[SELECT Id,Name,Participant__c,Participant__r.Family__c
		                    FROM Participant_Contact__c
		                    WHERE Participant__r.Family__c = :family.Id]) {
		    treeCon.get(contact.Participant__r.Family__c).get(contact.Participant__c).add(contact.Id);
		    contacts.put(contact.id,null);
		}
		
		incomes.putAll([select id,name,Income_Type__c,Income_Frequency__c,
					   Verified__c,Annual_Calculation__c
					   from Participant_Income__c where id in :incomes.keyset()]);
		
		contacts.putAll([select id,name,Contact_Type__c,Contact_Information__c
					   from Participant_Contact__c where id in :contacts.keyset()]);	
					   						   				   
		participants.putAll([select id,name,First_Name__c, Last_Name__c,
                			Middle_Name__c, Gender__c, Total_Income__c,
                			Social_Security_Number__c, SSN_Status__c,
                			Age__c, Disabled__c, Education__c,
                			Ethnicity__c, Food_Stamps__c,
                			Health_Insurance__c, Marital_Status__c,
                			Preferred_Contact__c, Primary_Language__c,
                			Race__c, Veteran_Code__c, WIC__c 
                			from Participant__c where id in :participants.keyset()]);
                			
		families.putAll([select id,name from Family__c where id in :families.keyset()]);		 
	}
}