TickingTimeBombx3 - I will post in a bit some links to websites that are about the acceptance/ non-acceptance of baptism and marriage between the Catholic and Orthodox churches. But as others have already pointed out, that is not really the issue. There is more of an problem that is not actually regarding this, but of the deceit that is perpetuating currently in your marriage and has been festering for 5 years now.
You ask "tips now on how to tell him if I decided to do that". That is very difficult to suggest how you might tell him. Perhaps think about how you would normally break bad news to him? I think you might find it helpful to write a letter to him, not to give to him, but it might help clarify what it is you want to say to him.
Things for you to think about:
Are you sorry for hurting him?
Do you, or do you not regret getting your first child baptised into your own church?
Do you still feel that you would "do anything" to get your second child baptised into your own church rather than wait till they can decide? (this you need to read some of the links I will put in a different post, Catholic view on original sin is not held in the same way as the Orthodox view, and there is not the same urgency to baptise and infant as is normally felt in the Catholic church)
How would you feel if by any chance his family have managed to get your child baptised into the Catholic church without you knowing?)
Do you want your marriage to survive?
What are you willing to do, in order to try to regain your husbands trust?
How will you prove to your husband that you will not do the same with your second child?
Think all these things through very carefully.
You will probably need some marriage counseling, how about this being with the Catholic service providers?????? How would you feel about this???
In the marriage counseling you may need to work together to decide how you want to support your children's religious and spiritual growth?
You will have to at some point tell your oldest child what you have done, you might find that they are happy you did it, or they too might be not happy that you got them baptised! You are in a difficult position, the sooner you get this dealt with the better.