Only a substantial change in circumstances allows you to petition for an increase, read 750 ILCS 5/510. If your order is old, you may be able to ask for an increase based on "cost of living" among other factors. But best if you either talk to an attorney or contact HFS to get specific guidance.
Child support should be set by the court. The fact that your child's father has another child by another woman does not affect your child support amount if your order was set first. Go see a lawyer and get your child support straightened out.
It sounds like you don't have a court order for child support and that you're just working this out outside of court. That's a big mistake.
The rule is the first child comes first. The rule won't apply, however, if you don't get to court.
Hire a lawyer.
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You should petition the court to set court-ordered support for your child. Everyday you do not bring a petition is another day you will not receive court-ordered payments, which may be higher than the $100 per month you are getting now. A person in your position should consider filing a petition immediately.
Your child support should be based on legal principals, such as the amount of income the non-custodial parent receives or the needs of your child. In addition and as other posters have mentioned, if your child is older and the support paid for your child should be calculated before calculating the support for the younger child.