Eloise, star of the popular children's books by Kay Thompson, is in hot water with the Internal Revenue Service. The IRS asserts that Julie Szende and John Hurd, Thompson's niece and nephew who administer her estate, had underreported its value by $1.7 million, and that the two owe about $504,000 in back taxes. Thompson's last book, Eloise Takes a Bawth (S & S, 2002), is a best-seller.