She needs harnesses back there to be safe. Probably the easiest thing would be to get tethers and use vests or EZ harnesses at those sizes.
If nothing is going to be changed then she cannot use boosters. It's more dangerous to use a booster without a shoulder belt than it is to just use a lapbelt only. And that's saying something, since a lapbelt only can cause spinal injury.
Here's a video of a child in a lap belt only versus one with a shoulder and lap belt in a booster.
[ame="http://www.youtube.com/watch?v=s3KXZQu4niA"]YouTube- booster lap only[/ame]
Is this the only car, or the fun car? If it's the fun car I'd park it for a while until she gets safe systems in place for transporting the kids. If it's the every day car I'd say lapbelt alone, but get shoulder belts retrofitted or anchors put in somewhere (EZ-On sells universal heavy duty anchors) and use a vest or harness every time.
Does the backseat have headrests? If not the kids can't sit back there because of whiplash risk once the tips of their ears go over the top of the backseat. If there are no headrests they'll need high back boosters to sit back there, which is more dangerous with just lap belts....
You can see it's not a great situation at all.
Wendy