Yes, you may.
One of the judges in the U.S. Bankruptcy Court for the Southern District of Texas recently ruled that to have a "confirmable" chapter 13 plan, a chapter 13 debtor must pay 6% interest on any "DSO" or domestic support obligation claim (which includes child support). In re Resendiz, Case No. 12-10603.
Judge Richard Schmidt analyzed the definition of "domestic support obligation" in 11 USC Sec. 101(14A) and found that it contained the terms "including interest." He also held that Bankruptcy Amendments Act of 2005 ("BAPCPA") had elevated DSO claims to first priority.
The Court found that the DSO claim is entitled to first priority status, and should be paid with interest. If it wasn't, the debtor could be subject to post-discharge enforcement actions by the custodial parent. He also cited the relevant Texas state law, Sec. 157.265 of the Texas Family Code, which provides that interest accrues on delinquent child support at the rate of 6% per year.
The Court also found that because all claims entitled to priority under 11 USC Sec. 507 must be paid in full pursuant to 11 USC Sec. 1322(a)(2), the Court had to deny confirmation of any plan that does not provide for such post-petition interest that accrues on a claim for DSO. So plans must provide for interest on such claims, in order to be confirmed. \
So, at least in Judge Schmidt's court, you must pay 6% interest on claims for delinquent child support. It's a good idea anyway, because if you don't, when your chapter 13 case is completed and discharged, the custodial parent could file an enforcement action against you.