There are a variety of reasons why child support can be modified - however, you can recalculate child support after 24 months. In your case, children have likely changed age categories and the schedules have increased by a legislative change a few years ago. Ex will have to provide tax returns. You can also look at his expenses which could give you good view of his income.
The above website has info on forms to make the request.
The new child support amount can be ordered only from the date the petition to modify was filed. You cannot and should not expect the new child support amount to be apply back to 1999.
Moreover, unless your old order of child support has already provided for post secondary education support, the child likely is about to go to college. You need to ask the court to order post secondary education support before the child is 18.
You should review the specific facts with your attorney to find out your legal options.
You should immediately either contact support enforcement and begin a modification action through them or file a Petition to Modify Child Support. As previously advised, the court or state will only go back to the date you start the process for any modification. The "statute of limitations" is not an issue - the last day to modify will be the last day the order is in effect, but that won't help you because there will not be anything to modify at that point.
The Petition is very straightforward. Instructions are available on site below. If you're not sure what his income is, guess high. If it is too high, he will respond and tell you or you get what you asked for if he doesn't respond. If you guess too low, he may not respond and at least you get what you asked for.
You can also get help through the Family Law Court Facilitator in your county.
You can modify child support as long as the original order hasn't expired under paragraph 3.13 Termination of Support. RCW 26.09.170 (5)-(9) provides the circumstances under which an order of child support may be modified, either by you or by DSHS. As for your ex's income, you can request his income information from DSHS using their form provided in the link below. This will only help you determine his income if he isn't working under the table.
This answer is not intended nor shall it be deemed to be the rendering of legal advise, it is given based on the information provided which is insufficient to give meaningful advise. This answer shall not be construed as part of the creation of an attorney-client relationship, nor shall it impose an obligation on the part of the attorney to respond to further inquiry. The Questioner has responsibility of obtaining legal advise from an attorney and is urged to do so.