I'm not an FA so these are just my own thoughts and experience, not financial advice....
The issue with most managed adventurous funds is they are often still rather conservative in their investment strategies, so not really adventurous at all.
Often a significant percentage of the fund is still invested in bonds. This means that growth is less volatile (it doesn't fluctuate wildly up and down in the short term), but in the longer time frame it often depresses the overall growth potential of the fund. Add the higher fees for the managed fund, and the comparison in growth to that of a simple index tracker over time can be quite weighty.
With a Junior SIPP for a 6 year old, the volatility is less of an issue over the time frame involved, because the fund has in excess of 10 years ahead to grow.
Take a look at this 10 year comparison chart from Trustnet, between:
Fidelity Adventurous Multi Asset Fund (C. yellow),
a Vanguard FTSE All Cap Global Index (B. red)
A low cost Vanguard S&P 500 index tracker (A. blue)
Fidelity 10 year growth = 91%
Vanguard S&P 500 index = 342%
You can see that both the Vanguard trackers are more volatile - the short term fluctuations are more noticeable; whereas the Fidelity is a much smoother ride.
But the differences in growth over 10 years are quite marked.
(Of course, the oft repeated phrase that past performance is no guarantee of future growth is an important caution to be noted).