Just to clarify a few things, as dermatologists might not be aware of the details related to pediatrics...
1. Although you must pass the general pediatric board exam in order to take the subspecialty pediatric board exams, you do NOT have to pass, take or even be eligible for the general pediatric boards in order "to continue in a fellowship", i.e. match into fellowship or do the fellowship. Most, but not all, pedi specialty fellows will take the general pedi boards during their first year of fellowship and currently, few foreign-trained residents who will not be pedi board eligible enter US subspecialty fellowships, but it is not an absolute rule. Also, you only need to certify general pedi once, after that, if you are also certified in a subspecialty you can recertify only in the subspecialty if you wish.
2. Pedi fellowships vary in their competitiveness from very non-competitive to moderately competitive. Although, as correctly pointed out by others, a poorly performing resident is not going to get their pick of fellowship areas and places necessarily, it would not be accurate to compare competitiveness for ANY pedi fellowship with adult cardiology or GI. I am personally unaware of any US resident in pedi who was "forced" to do general pedi for a career because they could not match into any fellowship. Note that depending on exactly how you count some areas including hospitalists, about 1/2 of pedi residents overall will do a fellowship compared to about 80% (I believe?) of IM residents.