When it comes to liability, the plaintiff's lawyer will go down to corporate records and pull the owners and directors of the company they are suing. They will then list the owners and directors as part of their lawsuit. If you are found liable, they can then seize any assets of any of those listed, so your plan of a holdco to protect you won't actually work. It will also expose your children to the liability as I said before.
I've got a buddy who's going through a company lawsuit right now and found out he's not protected at all by the corporate wall.
Again, 20-30k to your oldest, especially if he gets education tax deductions, could be paid as income and probably come tax free. If you pay dividends, each child would have to have different class shares, otherwise you have to pay dividends to all. You issue dividends on a class of shares, not on an individual holding shares. Everyone holding the same class, must get the same dividends/share.
As for making it an operating company, there are criteria that have to be met...flipping 1-2 houses a year may not qualify it as operating...I'm not a flipper, so I don't know what the criteria is.
From a finance state, starting a new company every few years means you start from scratch with the banks, CRA, etc. This can cause you problems where a track record could be an advantage...