My 4 year old takes zyrtec 1 tsp and singulair 4mg every night. She got a couple mosquito bites yesterday and they are swollen. Can she have a dose of benadryl also tonight? Her left eye has also been watering for several days even with both allergy meds.

Allergy. a one time dose should be OK. Diphenhydramine is short acting while Zyrtec is long acting. So, there will be some overlap, but short lived. Adding Zantac (ranitidine) also helps improve the symptoms faster since the two groups of medications have a synergistic effect. .
Single dose. it is in the same class but a single dose of Benadryl (diphenhydramine) is ok tonight . For the eyes you may try over the counter Zaditor eye drops .