That's hard to gauge. It's possible the 4Kids series broke 2 million at some point in it's run on Saturday mornings and Cartoon Network. Being on network television should have made that an easy task to accomplish but because TMNT 2003 started when Saturday morning ratings on network television were dropping dramatically it may have never even managed 2 million total viewers.
The 2003 cartoon may have actually done better when it was on Cartoon Network. I'll poke around on some ratings sites and such for some kind of comparison but we wont know the kid's ratings for TMNT until later this week or whenever Nick puts out a PR about them and I imagine they will if it did well.
Edit: Here's what I've found so far
The earliest info I can find at the moment is the 2003 series in 2006 was getting 1.3 with kids 6-11 (roughly 300,000 viewers 6-11). That's chump change to Nickelodeon. I wish I could find some 2003 or at least 2004 info when the 4Kids series was doing a lot better than that, at least in theory. There 2 hour Miguzi block ratings from CN back in April of 2004 averaged about 600,000 kids 6-11 (a rating of 2.6). Ratings ratios differ per demographic and we are only seeing the total viewer numbers for Nick TMNT so far and the ratings info for the 2003 show was mostly just for 6-11 but IMO it's the 6-11 that we need to see for Nick TMNT to gauge how well it is doing in comparison. I would be shocked to see TMNT get less than 1.5 million 6-11 and probably about 3 million total viewers.
Second Edit: More ratings
http://www.thefutoncritic.com/rating...0201cartoon01/
In January of 2005 the Miguzi average was up to around 700,000 kids 6-11 and the 2-11 was above 1 million so the total viewer ratings for TMNT was assuredly above 1 million but maybe not even 2 million.
Third Edit: Finally something from the FOX BOX premiere ratings
http://www.ign.com/boards/threads/tm...ings.34715212/
3.6 with 9-14 boys and a 1.0 with 9-14 boys is only about 120,000 so it got about 430,000 boys 9-14 on it's premiere. The 6-11 kids ratings were likely close to 1 million. These may not be the best ratings the 2003 series ever got but if it's any indication the Nick show is already off to a better start.