Help with kitten's innapropriate pooping

We have a 14 year old cat who has always been very affectionate to us and accepting of other cats. We babysat another cat several years ago and he was quite welcoming. So we recently got a new kitten. We gave it a lot of thought and we did think maybe he's too old to deal with this. However our 6 year old daughter has been harrassing our old cat quite a lot - wanting him to snuggle with her etc. and he really just wants to stick with us (his parents). We ended up thinking maybe a kitten would be good to take the heat off him a bit! So my daughter and I went to a shelter and chose a lovely kitten who was noted as loving cuddles and other cats. She is 5 months old. She is very affectionate and and seems to have settled in well. We have set her up in the spare room/playroom with her food and litter box and kept her just in that room for the first couple of days. We are slowly introducing the two cats and apart from a bit of hissing they've been okay so far. So the problem is, the lovely new kitten, Suki, knows how to use a litter box. She uses it often. Unfortunately she also poops all over everything else, cushions, rugs, sofas. We are at work for 8 hours in the day and she seems happy to be safe in her room (we don;t want to leave the two cats free to roam the house together) and she never has any accidents when we are not home. But it seems like the minute we get home she decides she must poop on something, so she has an "accident". I'm a bit stumped. It's not a matter of training because she knows how to use the litter box, it seems deliberate. Last night I was right there and she was in the litter box and then jumped out to poop on the floor. I realise it's most likely the stress of a new home but I don't want it to become a habit. We are letting her roam free in the house when we are home and don't really feel like she should be confined to one room the whole time. Help?!?