A notarized agreement is not a substitute for a custody order. Written agreements between parents on custody may work out if the parents are both committed to making an arrangement that works and have little conflict over the agreements. The agreement may show good intention, but it is not enforceable. It may seem like having it notarized would make it more official, but it really does not make a difference if the parties are not willing to do what they promise. That is why there is a court procedure for securing and enforcing custody orders.
if you do not have money to pay for an attorney, you could try to see if you are eligible for legal services. Another option is mediation, where you and the father pay a mediator to help you work out an agreement which you can then decide to enforce or not. You might have to pay to file a custody action and for the filing of the agreed-upon order. Generally, this would be less expensive than filing a custody action with the court.
All custody orders are considered modifiable, that is, they are subject to change, because the courts are looking at the best interests of the children and that can change at any time.
I hope you find a solution to your problem. Good luck.