peeing inside

so we have two shih tzu/pom mixes (allegedly lol, they look so different). Buddy came first, he's turning 3 this year. when we got him, he potty trained easily and would go outside to pee every time. then we got Barney. he turns 2 this year. when we first got Barney, he was using potty pads indoors because it was cold and rainy outside for a long time and my dad (whose dogs they are, pretty much) was sick. Buddy regressed a little because of that. once the weather cleared up, we started really working on house training. we have tried so many different things to try to break them of their habits. we've removed or replaced soiled furniture, tried enzyme neutralizing cleaner, "don't-potty-here" spray, "do-potty-here" training spray outside, rewards and praise, catching in the act and booting outside; we let them out 50 times a day to potty, they spend multiple hours roaming the backyard and peeing on everything out there. then they come inside and pee on my chair. they pee on a box if it's on the floor. tonight they peed on my daughter's brand new fucking tunnel. and the thing is, they don't do it when people are around! they fucking KNOW theyre not supposed to do it, so they wait ubtul we all leave the room. we have installed a baby gate so that they cant be left alone but my dad is 73 and forgets to close it sometimes. I'm getting so sick of these dogs, and of having to clean and sanitize everything and shampoo my rugs on a weekly basis. I don't want my daughter to end up crawling over piss. how the hell do I get it to stop? do we need to hire a trainer to come and help? I should also add that they are intact. I would love to get them fixed but my dad is convinced that it wouldn't help anything 😑 he's got that old man mentality that getting them fixed would "take away their manhood" or what the fuck ever. please help me because I am at the end of my fucking rope. l