If your onset date was 6/2008, you will then have a 5month mandatory waiting period before benefits begin. That is that point from which your backpay will be calculated. If you filed an application for your children in your initial filing you may have nothing further to do. However, in lenghty cases sometimes, the childrens app falls off and you will have to either reapply or apply to be their representative payee (which you may have already done). Your children will receive benefits as long as you have earned enough work credits to have them be eligible. If you got a yearly statement from SS it should outline approx how much you would get. Some people earn so few credits that their benefit is already at the family max meaning that even if approved, the children would receive nothing.
If your 18 year old is still in secondary school, they will receive benefits until graduation (assuming they are eligible). If not, you will receive backpay for up until they turned 18. I have a similar situation where my son was eligible when I became eligible, but he is now 19 so he will not receive a monthly benefit - just the retro backpay.