Question:

Should I tithe on child support?

The LORD spoke to Moses and told him to say this to the Israelites:

Leviticus Chapter 27 Verses 30 to 34 (King James Version)
And all the tithe of the land, whether of the seed of the land, or of the fruit of the tree, is the LORD's:
it is holy unto the LORD.
And if a man will at all redeem ought of his tithes, he shall add thereto the fifth part thereof.
And concerning the tithe of the herd, or of the flock, even of whatsoever passeth under the rod, the tenth shall be holy unto the LORD.
He shall not search whether it be good or bad, neither shall he change it:
and if he change it at all, then both it and the change thereof shall be holy;
it shall not be redeemed.
These are the commandments, which the LORD commanded Moses for the children of Israel in mount Sinai.

Jesus Christ said the following statements:

Luke Chapter 11 Verse 42 (King James Version)
But woe unto you, Pharisees!
for ye tithe mint and rue and all manner of herbs,
and pass over judgment and the love of God:
these ought ye to have done, and not to leave the other undone

Luke Chapter 11 Verse 42 (New Living Translation)
What sorrow awaits you Pharisees!
For you are careful to tithe even the tiniest income from your herb gardens,
but you ignore justice and the love of God.
You should tithe, yes, but do not neglect the more important things.

Luke Chapter 8 Verses 9 to 14 (King James Version)
And he spake this parable unto certain which trusted in themselves that they were righteous, and despised others:
Two men went up into the temple to pray; the one a Pharisee, and the other a publican. The Pharisee stood and prayed thus with himself, God, I thank thee, that I am not as other men are, extortioners, unjust, adulterers, or even as this publican. I fast twice in the week, I give tithes of all that I possess.
And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner.
I tell you, this man went down to his house justified rather than the other:
for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted.