Start off EASY.
Print out a basic will from online that denotes where your assets get distributed, and 2-3 levels of whom shall care for your children (my parents James & Penny Bond, if they are deceased, unwilling, or unable; my Aunt & Uncle Fitzwilliam and Elizabeth Darcy; if they are deceased, unwilling, or unable, etc.,
One can also state that under no circumstances are my parents Clyde Barrow nor Bonnie Parker to attain full or partial custody nor guardianship.
State that all your assets are to be awarded to your children.
Sign. Date. Spend $15 and get it notarized.
NOW put aside some money each month, and hire an attorney to do them up proper (with trusts -of various types, education, upon reaching majority, for their upkeep and raising, disbursements, annual payments to guardians &/or executor/executrix etc.)
AKA, better something than nothing... and then lawyer time.