Looking at the individual cards directly, no. Cards are intended to play together - when Creatures develops cards, they plan specific combinations such that they work well together given the other combinations out there. For example, one major deck currently is Lost Box, one version of which has zero V or ex cards in it - and still could win a major tournament, because it works as intended as a deck.
Looking at premade decks (that you can buy in a store), this is possible to some extent as the different classes of deck are meant to be played together. League Battle decks are of similar strength (and are based on ‘meta’ decks that could win major tournaments), V Battle decks or their kin ex battle decks) are weaker but on a similar level, and the Battle Academy set (which comes with three decks) is its own level.
If you go to a local league, you're best off determining what level they're playing at. Some leagues are very highly "meta" - meta meaning "the decks that are popular at the highest levels of competition" here. If that's what they have, then you're unlikely to be competitive until you make a deck that's at that level. You can look on Limitless to get a sense of what is popular online (which usually mirrors in person to some extent). As of the moment of this answer, any of the top 10-12 decks would qualify here; things like Mew Fusion, Miraidon, or Chien Pao Baxcalibur are relatively easy to learn and not very expensive.
Some leagues are less meta, and tend to have more variety in decks. Limitless still will be somewhat useful here as you can look further down the list and find more varied decks that perhaps aren't likely to win a major tournament, but could be fun to play and easier to obtain the cards for. As of the moment of this answer, decks like United Wings, Meowscarada, Zoroark Box, or Regis are fun decks that qualify in this space.
Finally, you may want to talk to the professor who is running the league, and see if you and the kids can get some help either learning or matching up with other similar level players. If your kids are relatively new, it is probably more helpful to learn the game in a noncompetitive setting first; many leagues offer something like that. The professor may also have example decks available to play that are intended to be at a similar level, and either the professor or another league member may be willing to teach you. It's also possible that this league is solely competitive, but they could suggest another one in the area that is more learning oriented - I work with a few leagues, and one is definitely 100% competitive, while another is a much better environment for kids to learn in. (If you're in the Bay Area of California, let me know and I can connect with you!)
A few footnotes that are more in the frame challenge space:
Pokemon decks do not cost several hundred dollars unless you want them to! In fact, the player that won Worlds last month did so with a deck that's remarkably similar to a League Battle deck from 2022. His deck cost just over $60, and odds are you could have gotten it even cheaper a few months prior. I built the basic core of this deck for several area kids, and it's very good and easy to learn. This is typical in Pokemon, unless you're playing cards that just came out in the last month; there are always highly competitive level decks available for $50-$100, and that is if you buy every single card for just that deck - 50%-70% of the cards in any given deck are typically reusable in other decks.
V or ex cards do not make a deck necessarily better. For the most part, in fact, it's the consistency cards that are by far the most important. Yes, often a V or ex will be needed to do the big hitting, but far more important are the Supporters (Professor's Research, Iono, Boss's Orders, etc.) and the Items (switch, battle VIP pass, nest ball) that make the deck work; you can build a similar deck with single prize cards that still is very competitive, so long as you pay attention to some basic things like 'how do I do enough damage to knock something out'. Lost Box, Zoroark Box, United Wings, all can manage without two prizers [Lost Box often does include a two prizer or two, but it doesn't have to], because they have consistency cards that make the deck work plus an interesting solution to "how do I do 300 damage".
