Both Squirtle and Charizard have amazing grab range. With Charizard's dash speed landing a grab really shouldn't be a problem. For Squirtle, you can try mixing up your jab combos with jab cancelled into a grab. Hydrograbbing can also help you land grabs - it's definitely something every PT should learn. As Nameless Wanderer said, empty shorthops into grab can be very effective too, thanks to the way Squirtle plays.
She has no grab combos really... d-throw -> up-b can work at low percents, but it's not guaranteed. When you grab with Ivysaur, just think about which character your opponent is, and where you want them to be. If they're easily edgeguarded (and Ivysaur's edgeguarding game is actually pretty good) and your back is facing the near ledge, you might consider a b-throw to get them off stage. Otherwise, a u-throw to get them above you is never a bad choice, since most characters won't want to be above Ivysaur.