Oral decongestants such as Sudafed may be a useful adjunct to anti-histamines in order to quickly open the nasal airway and dry up secretions. Antihistamines alone usually take longer to achieve these goals.
I don't recommend buying the antihistamine-decongestant combos for three reasons:
1) You won't always want to use an oral decongestant with the antihistamine;
2) If you need a decongestant, topical decongestants such as mild Neo-Synephrine or Afrin work more effectively than oral products; and
3) Oral decongestants only work well for 9 days in a row as the supply of neurotransmitters that mediate their action becomes exhausted.
When recommending therapy, I usually begin with the simplest first. I suggest a daily dose of Claritin selecting that over the others as it has fewer side effects. If that is insufficient, I recommend switching to the Zyrtec or Allegra. Even if the nose is the only target, a daily dose of oral liquid or a pill is still the easiest for most and particularly for children.
For stronger oral therapy, your doctor or your child's pediatrician may prescribe Singulair or oral corticosteroids. Singulair is a specific leukotriene blocker that works in tandem with the antihistamines. Corticosteroids effectively block most mediators of the allergic reaction. I mention these and other prescription only medications so that you will see how they fit into the treatment plan and know to ask for them when necessary.