A range of new ticketing options were introduced on 22 April, including a ticket for fourto 16-year-olds that will cost £2.50 if purchased online and £3.50 at the gate. Children aged 16 and under accompanied by an adult have previously been admitted for free.
A family ticket for two adults and two children now costs £34 (£32 online) - one adult and two children £19 (£18 online).
A Kew representative said the cost of the children's ticket is "modest relative to other top paid-for London visitor attractions" and has been reintroduced at the same price it was more than 14 years ago.
Kew added that the charge affects less than a third of children coming to the gardens because the vast majority visit through the schools programme and unlimited year-round entry for two children is included in family membership packages.
Aberglasney Gardens head gardener Joseph Atkin said Kew has "pretty much the same conundrum everyone has". He added: "You want to open to as many people as you can and you don't want it to be elitist - and you want to get the next generation into horticulture. But you have to find a way to pay your bills."
Hever Castle head gardener Neil Miller said: "I'm surprised they've managed to keep it free for so long."
Trentham Garden head of garden and estate Michael Walker said gardens need a "business model that is financially supported".
He added: "It's important that gardens can operate in a way that is commercially viable." Trentham's family ticket sells "more than anything else" and there is a "big opportunity" for Kew in selling a family ticket.
Professional gardening consultant Alan Sargent said: "It's a great pity, but I wouldn't criticise them. It's the wrong message at the wrong time, but it's a catch 22. It's either that or put the charge up for adults."
He added: "They have such a big shortfall they have to make it up somehow. Gardens tend to want to encourage children to come and often have specific offerings for them. That's what we need more of."
Ticketing - Queues prompt systems update
Royal Botanic Gardens, Kew is changing its ticketing systems after concerns were expressed on social media about long queuing times this spring.
The west London venue's new ticketing structure offers an online discount to visitors who buy tickets in advance. It plans to copy other attractions in the capital by issuing live waiting times and advising on which entrance to use via its website and social media.
Kew is also investigating mobile technology to enable it to take payments along the queue and will be introducing contactless payment "in the very near future".