Get your visas sorted before you come to Thailand but if you don't have a residence visa for Egypt you won't be able to apply there. Your husband can apply for a 90 day Non-O visa based on being over 50. You could possibly apply for a 90 day Non-O visa as his dependant as could his kid up until he's 20, but this depends on the consulate you will all apply at. You can't do this inside Thailand. Depending on his pension income it might also be worth looking at the LTR Wealthy Pensioner visa in which case dependants aren't an issue
If you already got your 90 day Non-O visa in Chiang Mai you must have a bank account with the required funds in it. It should be a simple process to get the 12 month extension of stay, just visit immigration and ask for their requirements