Dash dancing to grab is a great way to land painful combos on Fox and Falco. Wait for the d-air, dash away, and grab them in the landing lag. Let Nana come back, then do the vicious chaingrab, combo, or infinite of your choice.
If they start playing platform games to avoid getting grabbed, here are a few things to keep in mind.
1) Back-air > you. If they're facing away, stay back or get in front of them quickly. It has a lot of reach and is really difficult to punish unless they flub it against your shield.
2) Your u-tilt > d-air. This applies both to Fox and Falco. Get beneath them, and - timed correctly, of course - you will outprioritize everything.
Learn to shield grab very well. With enough practice, you can tell when the Fox or Falco messed up their fast fall, and you can land a grab. There are very few Foxes and Falcos in this country who can consistently and truly beat the shieldgrab.
But when you come against somebody who does, don't fight them when they're up in your face. You want to use your light shield to get pushed back, giving you breathing room, or you want to roll away. If you roll, you can set up a desync and possibly net the Fox or Falco in a blizzard, or you can have Nana throw an iceblock at them to occupy them. Force them to do something that isn't directly related to attacking you.
If it's a Falco, practice powershielding his lasers. Believe me, it's very, very doable as long as you focus. And you don't need to powershield 100% of the time either. Do it enough to make the Falco wary of using his lasers up close. This gives you a huge advantage as he approaches. He won't want to get stunned when he's coming in for his dial-a-combos, so he'll start trying to play it safe. Wavedash in with your shield up to confuse his spacing, and if he tries to get around you to get his distance advantage back, hit him with an up-air or jab and pummel him.
Falco can also have a bit of trouble with ice-blocks, because they control where he can and can't laser. Even if the ice-blocks aren't hitting, if they keep him from lasering your face off, then keep doing it. Ice-blocks have a lot of lag though, so don't use them as he's getting close (unless you plan to de-sync Nana and use her as bait).
Against Fox, he's going to shine you. A lot. He also wants very badly to gimp Nana, so if he gets her off the stage, he will try to shine spike her after her double jump. But you can make him afraid to do this by getting close enough to scare him. Threaten to b-air him out of his double jump, or grab him as he lands. If he's just sitting on the ground, waiting for you, then camp right outside his attack range. Wait for him to make some motion, then punish it. If he tries to smash, land a grab. If he goes for Nana or starts dash dancing, pressure him with a jumping blizzard. Once Nana reaches the ledge, she will probably try and ledge-attack him. If he shields, grab him. If he jumps, get beneath him with u-air and u-tilt.
Okay, the post was really messy, but I tried.