Enforcement of Multiple Support Orders on One Payor

My question is in regards to Multiple Support Orders on ONE PAYOR in Ontario. I'm separated from my husband. We have a signed separation agreement outlining his support obligations to me for our two children who live with me. We did not have to go to court for this, just went through our lawyers to get it executed. Prior to our marriage, he had two other children from a previous relationship in which he has an existing court order in place for support obligations. This court order predates our separation. My exhusband was very good at paying his support for the first few years we were separated, however, over the past few months has started to be very spotty in paying so I'm considering getting a court order based on our separation agreement so that the Family Responsibility Office will collect the child support for me. My question now becomes: how does FRO handle multiple support orders? Do they collect and pay chronologically? And what happens if they don't collect the full amount monthly of the combined two orders? Will this mean my children (for lack of better terms) are second in line to be paid their support simply because his other children had a support order in place FIRST? Another complication is I believe he is in arrears to his first support order in excess of $15,000. Would FRO put any funds deducted from him to the arrears on the first order first? I realize that getting an order would mean interest would start accruing on payments he doesn't make and FRO would put any of my support amounts to arrears as well for collection in the futue. But in that situation it could take YEARS to get funds and I need the funds NOW to feed and clothe my kids. To add insult to injury, the mother of the children for whom the first support order is in place is on Ontario Works (welfare) so has assigned her support to them, meaning SHE gets paid whether he pays his support under her order or not as OW sends her a cheque for support amounts on a monthly basis. Myself, I'm employed full time and don't have this option. Worse yet, I make more money than would qualify me to get legal aid assistance but not enough to pay a lawyer. Would really love some advice here on how to tackle this.