Birthdays

Simply put, what should I do? I have one child living with me and two of hers.

My sons bday is coming up and I going to spend roughly 300 on it, she is going to make the cake and help set up, but paying 0 towards it and not buying any gifts, the gifts I buy will be from both of us.

I don't want to foot the bill for both of her kids bdays. I feel, just like me, she should pay 100% for their birthdays and I'll help and get each of them are gift, but not anything close to 300.

Should I pay 600 on both her kids birthdays combined to justify spending 300 on my son when she is not paying anything for my son?