the souvenirs disappear because the previous parent left with a few of the souvenirs to remind them of their child before they leave to tama planet. the technical reason why the souvenirs disappear is because of a glitch(other glitches include the balls in the food section, and screen problems). i don't know why your tama does the disappearing grandparent, but when i was on my 5th gen i could see my parent, grandparent, and even my great grandparent. some tamas just dont show the grandparent. and after you get the cell phone with just the parent, the parent disappears. so if i were you i would wait until you have the grandparent(meaning say if youre on the 6th gen, you would wait until the 8th gen to get both souvenirs, but do the grandparent first, otherwise both may dissapear before you get the bike.