As well as each adult needing to meet the financial requirements they will each also have to satisfy the criteria they apply under. The adults are completely distinct applications. The kid can apply after a parent has been approved and can be sponsored by that parent
My drawing skills are dire too. A number of years ago one of the immigration officers took my rough map and redrew it so I just use photocopies of that now. He wanted a fair bit of detail, the complete 30km route from where I live to immigration including all junctions and traffic lights plus some recognised landmarks
You file your report where your TM30 is registered. But if you've just arrived in Thailand you don't file until you've been in Thailand for 90 consecutive days. Also if you've just entered you need a fresh TM30. If you want to file in Chiang Mai you need to have a TM30 registered with that office