In the bathroom...
Both Dipper and Wirt came in there wearing swim trunks. They knew that they'd get wet, so they had taken precautionary measures. "Alright, Lisa. Bathtime!" Dipper exclaimed happily, putting her in the tub that was filled with several bubbles. Wirt had the temporary job of holding Lisa's pink pajamas and towel. Then he was relieved of it when Dipper took the stuff from him and put it on the counter. Wirt then handed the baby shampoo and body wash to the younger boy, who told him thanks. He then knelt down with Dipper and held Lisa. She started to mess with the water, a few drops almost getting Wirt in the eye.
"Come on, Lisa. You promised to take a bath, remember?" Dipper chided, which made her stick her tongue out at him. "Don't make me use the tickle monster on you." He said in a somewhat menacing but silly voice. Lisa's eyes widened, knowing that the tickle monster was her weakness. Wirt snorted a bit at her reaction, though he contained himself when Lisa glared at him. Dipper raised an eyebrow at the taller boy, but decided to continue by putting the baby shampoo in her hair, getting it all nice and soapy. She picked up some bubbles from the bath with her powers and gave Dipper a bubble beard.
"Thanks Lisa! I always wanted one! Wirt, how do I look?" He asked the other. "Like Santa Claus." Wirt replied, trying not to laugh. Lisa grinned proudly. Dipper brushed the bubbles off his face and put a few on Wirt's nose just to be funny. The older boy blew the bubbles off. Dipper chuckled and covered Lisa's eyes to wash the shampoo off, which made her giggle. Then he got the body wash and poured a little onto a shower sponge and started to rub Lisa's body with it, getting it very sudsy. "T-That tickle!" Lisa squealed out. Dipper just smiled teasingly and continued scrubbing her body with the shower sponge.
When he was done, he washed it off and turned to Wirt saying, "Alright, Daddy. Time to dry the baby. Hand me the towel, please." This made the other blush embarrassingly, but he got the towel and gave it to Dipper. He then took Lisa out of the bath and started drying her up. Then he got the baby pink pajamas and put them on her. "Yay! I clean!" She exclaimed, raising her arms in the air happily. Dipper smiled and picked her up, saying, "Yes, you are. Now let's go get those two cookies i promised you." Then he walked out of the bathroom with Wirt in tow.
After both boys changed into different clothes, Dipper headed for the kitchen and got two chocolate chip cookies for Lisa. She took them gladly and started nibbling on one. "Gosh, you're so adorable." Dipper said, ruffling her hair a little. He went into the living room and sat down on the couch with Wirt and Lisa. She started to eat the second cookie, but was also getting a little sleepy. Dipper hummed a lullaby and rocked her to help her go to sleep, which she soon did. Wirt sighed in relief and said after a moment, "Taking care of a kid is hard. I should know, I had to help take care of Greg when he was born."
Dipper laughed a little and replied, "It's okay, Wirt. At least she's asleep now." He then laid his head on the older boy's shoulder, which made him blush red like a tomato. But he didn't mind. They both were getting sleepy themselves, so they laid down on the couch, with Dipper holding Lisa carefully and Wirt wrapping his arms around him. "Goodnight Wirt." Dipper said, giving the other boy's cheek a kiss before falling asleep. "G-Goodnight Dipper." Wirt replied, kissing the boy's head and closing his eyes.