When should I ask the judge if I can claim both of my kids since they live with me 90% of the time now?

You can petition the court for whatever you would like, problem is, will the Court grant.

Generally, you might be able to ask for your attorneys fees or costs based upon the fact that he brought a false allegation. As to the allocation of the exemptions, unless modified the order will control. This will be doubly difficult change if the original judgment was entered by agreement or stipulation, commonly called a Consent Judgment. The thing which could potentially make the difference is if he was also ordered to pay, as part of the judgment, child support and has failed to meet that obligation. It also might be a good time to seek a reinvestigation of the support obligations depending on changes in income since the time that the judgment was placed if you believe that he is making more presently than he was at the time of entry. You can see from the above this is not typically delete simple matter in the sense that there are additional issues that you should be taking into account and therefore I would suggest three strongly that you consider engaging attorney to represent you, and hopefully transferring most of the fees to him because of his false allegation.