Like Christy said, the bottom slots are 10", and even with the newborn padding most newborns won't fit well for several months. Britax changed their recommendations a few years ago. It used to be ok for the bottom slots to be up to an inch above the shoulders. But now they say that the shoulders have to be even with the bottom slot. So even a long torsoed freak of a newborn like Piper wouldn't fit in a Britax at birth anymore (she had a 9" torso and was 1" below her Advantage's bottom height).
If they want a convertible from birth there are several to chose from. First on my list would be the Learning Curve True Fit. It rear faces to 35 pounds and forward faces to 65 with 17.5" top slots. It has tons of infant padding, has a removable headrest to 22 pounds/1" from top of short shell, so it's a tiny seat for a newborn. Easy to get a 45 degree angle and still not eat the dashboard. It's got built in lockoffs, a seven year lifespan, more leg room than the Boulevard, a taller shell, taller top slots, is just about as easy to use, and costs quite a bit less ($120+ less).
After that the new Graco My Ride and the Evenflo Triumph Advance are tied on my list. The My Ride has a 40 pound rear facing limit (woo hoo!) and a 65 pound forward, but only 16" top slots. The shell is about comparable to a Boulevard. It fits newborns really well, reclines very easily. I haven't installed it yet. It's $150. However, it looks like it may be hard to get more upright as the baby gets older, and it's heftily reclined forward facing as well. For a backseat like mine that's super sloped that seat would have Piper nearly at a 45 degree angle forward facing. I don't think it looks comfortable. But the rear facing weight limit, decent top slots and great forward facing weight limit and shell height, combined with a nice price tag, put it high on my list. The Evenflo Triumph Advance is $140-$180, has a 35 pound rear facing limit, 50 pounds forward, and 17.5" top slots. It has knobs to adjust the harness, which people tend to either love or hate, nothing in between. It offers a lot of leg room and has a very unique harness system so you never have to rethread it. It's a big comfortable bubble of a seat. Pretty easy to install and use. The worst part of the installation for me is always tucking the cover back down. I can install it in about 30 seconds, then spend 10 minutes on the cover.
Next on my convertibles for newborn list is the Sunshine Kids Radian. It's big front to back, can be a bear to install, but it's got a great angle for newborns, low bottom slots, an eight year lifespan, nearly 18" top slots, 35 pounds rear and 65 pounds forward (or 80 pounds, if you want to spend the extra money, but you don't get extra shoulder room), it folds for travel. It has a removable but required rear facing foot. It's a nice, solid seat with quite a few quirks. It runs about $210 for the 65 up to I think about $280 for the XT or 80 pound versions.
The last on my list would be the Cosco Scenera. It's a barebones seat, but if they have their heart seat on a Boulevard what may work best would be to get this seat, use it until baby fits into the Boulevard, then use this as a spare/travel seat. It's lightweight, narrow, easy to installl, and at $45 if it sits in the closet half the time, who cares?
Wendy