When a desperate father loses access to his only child, he attempts to take matters into his own hands, only to find himself taken hostage by his vindictive ex-wife.

This is a great start, and it does pique interest. If fleshed out properly this could make for a very great screenplay.

"he attempts to take matters into his own hands"

I feel this is too vague as well......what "MUST" he do?

"taken hostage?"

Again, what does this mean.....physical, mental...psychological?

Feuding parents has been done before, and very well (Kramer v Kramer for one), I tend to agree with DPG the kidnapping parents adds a hook of interest. I suggest re-thinking this concept with the idea that the mother taking the father is the inciting incident. I'd also raise the stakes and make it more than one child.

For example:

After being taken hostage by his ex-wife a father who lost custody must break free to save his children.