Technically, one who wishes to drink after the first of the four cups may do so, but le-khatḥila it is best not to drink between the first and second cups, lest one become intoxicated to the point of being unable to read the Hagada with proper focus. However, it is permissible to drink non-alcoholic beverages such as grape juice or any other juice (SA 473:3, MB 16 ad loc.).
If, while reciting the blessing over the wine, one had in mind to drink other beverages, he need not recite a berakha over them, because the berakha over wine covers all beverages. Even if he did not intend to drink other beverages, but they were on the table and there was a chance that he might want to drink them, he need not recite a berakha before drinking them, because the blessing over wine covers these too.
A firstborn who fasted on Erev Pesaḥ, or someone so hungry that he finds it difficult to concentrate on reciting the Hagada, may eat foods such as eggs, fruits, potatoes, and kitniyot (for those who eat kitniyot on Pesaḥ) after kiddush. However, he should not eat too much, so that he saves his appetite for the matza. Furthermore, this is only permitted when there is great need. If one can restrain himself, it is best not to eat anything before the meal, because when one eats after kiddush, he runs into the uncertainty of whether he has to recite a berakha aḥarona (and if one eats, he must recite a berakha aḥarona).
This eating and drinking is only permissible before one pours the second cup and begins reciting the Hagada. However, after beginning the Hagada, it is forbidden to interrupt by eating or drinking (BHL 473:3, based on Ramban and Ran; however, it notes that Ha-ma’or and Tosafot permit).
. One would presumably need to recite a berakha aḥarona. Since the food one eats before a meal is not considered part of the meal, he must recite a berakha after finishing the food before beginning the meal. This is the opinion of Ben Ish Ḥai (Naso 4), Kaf Ha-ḥayim 177:7, and Or Le-Tziyon 12:7. In principle, MB 176:2 also rules this way, but states that according to several poskim, if the food is of a type that is not covered by the berakha of “ha-motzi,” such as fruit, and the person intends to continue eating fruit during the meal, he should not make a berakha aḥarona, since the berakha that he made on the fruit will include the fruit he eats during the meal. Also, since Birkat Ha-mazon covers the fruit that he ate during the meal, it will also cover the fruit he ate before the meal. Consequently, an uncertain situation arises, since according to Rashbam the berakha on the karpas is also supposed to cover the maror. Therefore, SA states that one should specifically eat less than a kezayit of karpas so that he will not have to recite a berakha aḥarona. If one recited a berakha aḥarona over the food he ate after kiddush, that berakha would cover the karpas, and the berakha he made on the karpas would no longer cover the maror. Nevertheless, in practice, one should recite the berakha aḥarona of Borei Nefashot and not make another berakha on maror, for in cases of uncertainty regarding berakhot, we are lenient.