If you add an adult child who does not live with you as a joint tenant on your principal residence - that will be considered a gift.
The gift is not taxable income for your child - regardless the amount - as a recipient of the gift your child does not need to claim it as income. There is no any amount limit. Please see for reference IRS publication 525 page 30 - http://www.irs.gov/pub/irs-pdf/p525.pdf
However you - as a donor she should file gift tax return - using form 709 - http://www.irs.gov/pub/irs-pdf/f709.pdf - if the value of the gift is more than $12,000 per person per year (for 2008); there will not be any gift taxes unless the lifetime limit of $1,000,000 is reached.
If the property to be sold and there are two joint tenant co-owners - each owner will be responsible for half of the gain.
You may shelter your half of the gain based on assumptions that you owned and lived in the property for at least two years out of last five before the sale (up to $250,000 if you are single).
Your adult child - as not lived in the property may not use this exclusion - and should pay capital gain taxes on his/her part of the gain.