I threw some round-about numbers into tax software to see how the refund would change.
With the estimated figures you gave me, I came up with: With 2 dependents (his children), filing Head of Household, he would get back approximately $10,306.
Filing a married joint return with you, claiming 4 dependents (your 2 children and his 2 children), you will receive about $18,281.
Please keep in mind, I am not saying your refund will be $18,281. I am showing you, without taking any possible education expenses (for you or your daughter), his refund will increase just about $8,000 if you two file a joint return and claim 4 dependents.
Please let me know if I can assist you further.